Friend A believes Friend B should try something before deciding not to like it.
I hope I don't come across as pedantic, but I think that your questions may contain what philosophers call "false alternatives."
First, there's a sense in which both A and B can be correct. It might be that B is well-advised to try a particular something before rejecting it because the risks associated with trying it are small compared to the possible benefits. Nevertheless, it could be true that B "shouldn't have to" try something before rejecting it: that is, B might well have the right to refuse to do X even if he would be well-advised to do X.
Second, C can help convince B to try the thing even while C lets B make his own choice. As I see it, giving B convincing reasons to make a particular choice needn't mean depriving B of a choice -- including a free choice -- in the matter.