Based on my own experience (though mine is two-way), a friendship between a t and a client makes for a convoluted, effed up, mess. The whole thing can be an absolute mind f#*k. Yes, the end result can be rewarding, but the road to getting there is so twisted and menacing it isn't worth it.

Best advice I can give you is to make sure the friendship does not become two-way while you are in therapy with this t. Also, if you haven't engaged in non-therapy related activities as friends, DON'T. This will help prevent any further entanglement. What helped me to gain better clarity around things with my t was to talk about her friendship intentions. This helped me to get a grip on the reality of the situation. A good way to find out your t's true intentions around the friendship is to ask what happens with the friendship when therapy ends.

Ideally, if I hadn't been so incredibly attached to my t during the time our friendship started, I would have sought out a new therapist in order to attempt to further pursue the friendship with my t. Would have made my life a thousand times less painful. I find there is darn good reason the therapists Code of Ethics asserts the therapist not engage in personal relationships with a client until some time after therapy has ended. Such friendships come at too high a price to the clients sanity and well-being, imo.