OK, there’s a rude way to say this and a nice way to say this. Personally, I choose the nice way, but I’m going to start with the rude way because although blunt it gets my point across quicker. If they’re working out on their own, it doesn’t mean that they know what they’re doing, especially if they’re not getting the results they want.
This is where offering to do a fitness assessment so they can benchmark their progress could be a good tool. They may be very fit in some areas, and not so fit in others. An assessment can help you identify their weaker areas. Almost no one who works out without a trainer is fit in all elements – strength, endurance, power, balance, flexibility, etc.