There was a great article posted here once written by a therapist as to why he won't even entertain the notion of being friends with former clients, even the ones he adored and got along with the best because of all the complications it brings up and how it impacts the Memory of their therapy.

What you get from a good therapist should be life-lasting. You should be acquiring tools that you will use the rest of your life. You should have good associations with those memories and TRUST in them. His opinion was that be a friend to someone after going through therapy with them endangers the lessons and associations with everything that happened in therapy.