First of all, good on you for being a good friend and caring about her
I would say maybe you should tell her your opinion on the kind of relationship she's having with Bill.
If she really likes him maybe she'd preffer to have a steady relationship, but since Billy's not up for that, maybe she settled with sharing him with other people - as long as she could have him herself too.
In my personal opinion that's not something I'd go for, but it's just another relationship pathern that can be done, as long as both parts are aware of what's happening and are extra careful when it comes to making out and/or having sex, because if he is sleeping around with other girls (and he might not be even using protection), he's more exposed to STD's, therefore she's at a higher risk.
Alert your friend to the fact that if what she wants is a stable relationship with someone, at the moment Billy is not the right person for her, and she shouldn't put up to with his "needs" just because she likes him. She must know that he's seing other girls and that she's not his only partner at the moment, and she should be able to accept it and everything that comes with it so they are in healthy relationship.
If she realises that's not what she'd like for herself, then if decides so, she should break up with him, just stay friends -if she wants to- but try to avoid anything more intimate.
Ask her if they're using enough protection (good latex condoms and even better if she's on the pill too so she doesn't get pregnant). If they're not, take your friend to do some STD's tests, and if she's not on the pill and not using any condoms or other forms of protection, get a couple of pregnancy tests and have her do one now.
To sum up, tell her what you think about the situation, ask her if she's comfortable with just a casual kind of relationship, and have her tested on STD's.
After that she can think of what she wants/need and if that's what she's having, and decide what to do from there.
If she needs other sort of advice/support or she'd like to talk about it with other people you can always introduce her to Scarleteen and we'll take care of her
In the meantime, good luck