I don't think girls and boys being friends really works. I don't have any female friends and don't want any. I wouldn't find it appropriate for my GF to be hanging out with a lad 1 on 1.
However, whether their relationship is appropriate or not, you kind of signed up to it if they have been this way for years. Your options are either put up with it or try to get him to distance himself from her. The second option could well end up with him choosing her over you.