The problem is, people can only talk generics here, because no one knows your female friend.
It could be a problem for her, or it may not be.
She may be quite happy to have you attracted to her, and at the same time not at all want you to make a move on her. Or she may not at all like the idea of you being attracted to her. Or she may secretly be curious about you.
There are of course many ways to bring the subject up - speaking about 3rd parties, movies with girl scenes, jokes, being out at a bar and letting her know another girl is attracted to her etc (unless you are too young for the latter).
And of course some girls will go for 3 somes where they wouldn't go for just the girl on girl thing.
Play it by ear, but I wouldn't put too much store in it. The most likely outcome of any sexual encounter between the two of you over the long term, is the loss of friendship.