I've had this discussion with a lot of trainers online. The short answer I'd give is it depends. This is a hazy issue.
The first thing I'd like to point out is that if a client wants to dramatically change their physique, they NEED to learn about nutrition and change their diets. There's no getting around that.
Should their trainer be the person they get their diet advice from? Depends a lot on the trainer, the client, and the trainer:client relationship.
I did 3 semesters of nutritional studies during my 4 semester Diploma of Fitness. As such, when trainers tell me that giving nutritional advice beyond the basic "don't eat like stupid person" is outside of my field of practice, I find that a to be a significant professional insult. FFS, we spent much less time learning how to test fitness or develop exercise programs or instruct exercises or deliver personal training sessions (you know, the stuff that we actually do in all our sessions) than we did learning about nutrition.
In saying that, different trainers have different levels of nutritional competence. I'm absolutely confident in my ability to write a diet plan for someone who has no medical issues, in accordance with their goals. It was a requirement for my Cert IV that I do this with a real client in order to pass my course and become qualified. I wouldn't, however, provide the same service for someone with a medical issue that could be effected by diet. In fact, it's unlikely that I would provide this service for someone who is healthy, because if I give you a diet plan, there's a dietitian out there who could do a much better job at it than I could, and there's a very high chance that if I did it you wouldn't adhere to it anyway. On the other hand, if I give you general nutritional advice related to your goals, and offer some tips on how to implement that advice, you can figure out a way that works for you to get your diet on track.
There are trainers with much better nutritional knowledge than me. There are trainers with much worse nutritional knowledge than me. To be fair, there are much better trainers than me, and there are much worse trainers than me. Should the trainers who are generally not as good at this as me just not train people? Or should they operate within their strengths and develop broader, deeper knowledge so that they can do better/more? Trainers shouldn't be giving out bad nutritional information. That's a given. And for plenty of trainers, that means they shouldn't give out nutritional advice, because they don't have the knowledge (hopefully they know they don't have the knowledge, so that they can know to either open a book or shut up and refer on).
But once again, if you're aiming to change your physique, which 90% of clients are after, you need to change your diet. Diet isn't an elective, it's mandatory. Since this is critical for clients to get results, as a PT I can't justify not having sufficient nutritional knowledge to be able to give sound advice to the majority of clients. I NEED to have this knowledge because THEY NEED to have this knowledge.
I'm not a trainer, nor have I been a regular client, but as someone who is long-term committed to fitness (method varies according to current goal/focus), who reads about food and nutrition for pleasure and knowledge, who regularly participates in forum discussions, and whose spouse does the same leading to many conversations at home, I have to say most PT's should not be giving advice (and what I overhear in my gym seems rather suspect although admitedly I'm not getting the whole "plan").
The thing is, the same thing won't work for every body and there is so much mis-information around. Nutrition is just not an exact science and the way one person responds to certain foods/macro's can be hugely different than another. For example, when a body is insulin resistant (and in the US something like 80% of overweight people and 30% of normal weight people are insulin resistant) the way that body responds to carbs is much different than someone who is not insulin resistant. How is a PT going to know, blood glucose isn't a good indicator.
A friend just signed up for a 3 month program with an online trainer and nearly everything I've heard so far just has me shaking my head in disbelief - for a 150 lb woman, this trainer is suggesting 155 grams of protein a day. What? It's just stupid. Guess we will see if that is right for my friend or if she just gags on all that protein.
Lifestyle and priorities are a huge factor so some general guidelines might be helpful. "Stop eating garbage" is helpful advice across the board I think, but many people can even debate about what is garbage. Just getting people to think about their food choices and consider what they are eating is a huge and helpful step for most.
In the instance of your friend, 1g protein/lb bodyweight/day is a fairly common rule of thumb. There are a lot of individual studies which refute the 1g/lb guideline, but there was also a meta-analysis done a little while back (I think it was about a year or two ago) which looked at all the seemingly contradictory information about the effects of protein intake. It found that the studies which said "more protein =/= better results" had such a small addition that it made perfect sense for there to be no significant difference, while the studies which said "more protein = better results" had a significant enough addition of protein to show a significant difference in results. And it turned out that 1g/lb worked out to be a pretty good dose for getting improved results without entering diminishing returns/problem territory.
In saying that, so long as you're consuming enough protein to meet your health requirements (which is only about 0.5-0.6g/lb off the top of my head), you'll be alright...but up to double that minimum standard does appear more effective for athletic development, at least based on this meta-analysis.
Of course, all sources about how much protein you should consume tend to come from someone trying to sell something. A really helpful resource I had back in my Diploma days (I can't remember where this information came from, unfortunately), prescribed something like 0.8-1g/kg for sedentary people, 1.2-1.4 for endurance athletes, and 1.4-1.7 for strength athletes. Vegan websites seem to have cottoned onto this information, but take the sedentary prescription and say that's what everyone should be eating. They're selling something. Up the other extreme end of the spectrum, the makers of Max-OT prescribe (IIRC) 53% of all macro-nutrients come from protein, which, on a 4,000kcal/day diet, is a bit. Did I mention that Max-OT is a program made by a supplement company? The groups that recommend 1g/lb (or 1.7g/kg) are busy trying to sell strength programs/equipment/gyms rather than special diets. Everyone's selling something. That's not necessarily a bad thing, but it gets you thinking about motives, and what's in it for the person giving the advice. For the trainer/gym owner, if they don't make a cent out of food/supplements and get paid on the basis of people achieving their goals, then maybe their incentive for recommending a certain amount of protein is that it will actually support your goals. They could still be wrong, but it's certainly more noble to try and sell something to someone so that things will work out for them than to try and sell something to them that will either hinder their goals or will do nothing to help them.
