"Normal" is hard to say. Does it happen? Yes. Can it get in the way of healing? Yes. But it really has to be his decision. Hopefully therapy can help him figure out why he continues to see them and if it is healthy for him. Once he understands the why, he will have a better idea of if it is something that is wise to continue. It will have to be a decision he comes to on his own. Changes that are made under external pressure don't tend to stick. I'd voice the concern to him once, but then after that let him know that you support him and believe in his ability to come to his own decisions.
Hope that helps.
"These days I just try to keep to myself,
well aware I've lost touch with everyone else.
I understand that I'm fading away."