This is most often referred to as assisted stretching. It is within our scope of practice. I teach a CEC course for this. It helps inexperienced fitness professionals to teach their clients how to stretch and to gain practical experience in learning to gauge the proper tension at which to assist a stretch,.
Yes a personal trainer can stretch a client.
The question is: should a personal trainer stretch a client?
a) A trainer should understand how to stretch clients and determine what
method suits each client (similar to what Michael said)
b) As Karin mentioned we do not ‘manipulate joints’. Our goal is to use
stretching techniques to improve ROM.
c) I agree with Paul…a trainer should ask their clients about any orthopedic
issues and have them see a physician (no need in making a situation
Your best bet …if you do not feel comfortable with your ability to stretch people, don’t do it.
Hope this was helpful…