Responses people have to such a situation boil down to two:
1. They do not let their feelings grow for him, as they have too much care and respect for their "best friend" - and themselves.
2. They query every friend and forum they can, in the hopes of finding a justification to act on their basest of impulses.
As always, they should remember this - if he can be stole from his current girl friend, he is not worth having, if he can't, then a friendship has been ended for nothing.