Why are people suggesting the need to choose between the two of them? Surely having one doesn't rule out having the other? If choosing him would cause the friend to refuse to be friends with her, then what sort of a friend would she be, anyway?
Anyway, I think you need to ascertain as to why she dislikes him, and then ask for his side of things, too; stories from one person can be too one-sided, obviously. Only then can an informed decision be made. Your friend may know something you don't, admittedly, but she may also be being silly and/or jealous, and you may be treating this guy unfairly, and sacrificing something special, if you stop seeing him.