For those of us in the U.S only...the APA (American Psychological Association) does not say it should not or cannot be done. Neither does the NASW (National Association of Social Workers). That certainly does not cover every therapist in the U.S., but it does cover a certain percentage of them.
So I imagine each case should and would be different. If a client wished to be friends with a T post termination, I would hope a T would determine if being friends with a client would be healthy for that client (and also for themself!) and make a decision from there.