As long as you stay within your scope of practice then you should be fine. The others have provided some excellent points which you can go by. I would also support the idea of getting a nutritionist or even RD certification/degree if you want to be more involved with your clients’ diets and meal prescription.
This is very much like the topic of online training for me. Clearly we are not properly educated to be making specific food advice. While many feel it is not a “big deal”, there are so many potential problems that could take a long time to be detected. And there is always the chance that you will recommend something that directly affects a clients prescription medication or causes an allergic reaction. Even though the odds are long, the mere fact that the possibility exists should keep us from crossing the line from our defined scope. We should always refer clients to the best possible person/professional to help them. In this case a registered dietician. While a trainer may have good intentions with including nutrition advice in their programs, in reality, many are just looking to increase revenue.
Just like online training, the best option for clients is to actually work directly with a certified fitness instructor. In some cases where that is not possible, I could see the potential to help them as being better than no help at all. But there are very few people that are so isolated that they need to contact a fitness instructor via the internet. Or worse, simply purchase a workout/exercise list with no idea of what is and isn’t dangerous or unnecessary for the client to be doing. Again, many times this is just a money scheme, not a sincere effort to help someone. And there is a difference between online training and giving a client exercises to do in your absence.
If you want to work in nutrition, get your RD. If you want to be a personal trainer, meet your clients in person. Being properly educated to do a job is what makes a professional in any field ethical.
The Nutrition Advocacy Group(nutritionadvocacy.org) will tell you exactly what you are allowed to do with nutrition in your state. In many states Registered Dieticians are legally the only ones who can use “nutrition tools” which include: health histories, dietary and lifestyle assessments, counseling, goal setting, monitoring etc. Nutrition Advocacy will let you know where you stand.