Contracted Psoas Major

Hi, I have a client with scoliocis and a very tight psoas. Her trainer is also a P.T.A. and suggest that she never does PNF stretching to it. He claims it will only worsen it. During my massage she will let me do PNF stretching, but refuses to do it any other time. He says stretching only loosens the joint opening it to injury. I disagree, what is your advice. Thank You!Gary