It does sound unusual. I suppose perhaps if she is seeing him for a very clear-cut thing (eg, help quitting smoking, or something) and maybe he chooses to socialize with several similar clients at his discretion and it just works?

I can't think properly tonight so I can't even offer a proper considered opinion on whether I think it's okay or not. But it seems to be working for your friend, so hopefully it continues that way. More than one road leads to Rome.