honestly dude, just say hi. start by saying hi, if you see her reading a book or something, ask her about it. little things like saying hi once in awhile can easily be turned into more whether it's her starting the convo or you. since your friend seems to know her, when your friend is around her, go talk to your friend and acknowledge the girl too. since your friend is friends with her it should make it easier for you because your friend can be kind of..a wing man? wing woman??? to help because your friend is comfortable with both of you. she knows you find her attractive though and doesn't seem to have been put off by that, she probably won't be put off by you saying hi to her or even starting a convo with her. the best thing about having your friend be friends with her is you can ask your friend what she thinks and if she thinks you're being creepy or forward or w/e, your friend will probably tell you.
just my advice though.