I would say that manual stretching is definitely within the scope of practice of a well-qualified (and certified) fitness professional. (I certainly hope so, I’ve been doing it for years!) However, you don’t want to overstep your boundaries here. If you have a client with orthopedic issues, it’s best to consult with their physician or physical therapist first – they’ll tell you what to do. The last thing you want to do is cause more problems. Like anything, if you’re not comfortable doing it, don’t.