It is sexual harassment. The first step to solving this, however, is for Bill to frankly tell her he isn't interested and doesn't appreciate her advances. This is the simplest way to go about it. As much as you may not want to seem rude, being adults, you just need to be frank with her IMO. Although you can talk to the manager about this, the first thing he or she is going to ask is whether Bill has talked to her about it.

It's hard to deal with people like this, especially because most people don't want to come off as rude, no matter how disliked this person is.

Good luck to you and Bill with this crazy lady!

I also agree, remind her of her fiancee, ask about her wedding. Encourage Bill to talk about in ladies he may be with.