My fiancée cannot use hormonal birth control for the same reason. Instead, she has a copper based IUD. Your girlfriend has probably considered this already, but if not I strongly recommend looking into it. They're not perfect, but they're very good.
(There are only two real risks/side effects: Heavier bleeding, and it's important not to get Pelvic Inflammatory Disease (which is another way of saying, it's even more important than normal not to get STDs. If she can handle the first, and is in a monogamous relationship - or at least, always uses condoms with people who aren't guaranteed STD free - then neither issue is severe.)
(There are only two real risks/side effects: Heavier bleeding, and it's important not to get Pelvic Inflammatory Disease (which is another way of saying, it's even more important than normal not to get STDs. If she can handle the first, and is in a monogamous relationship - or at least, always uses condoms with people who aren't guaranteed STD free - then neither issue is severe.)
Mind you, vasalgel sounds even better to me. :)