If she's going out and buying tons of magnums with no boyfriend then obviously that's a huge sign of a slut. I'd call it a red flag, not a deciding factor. If she's hooking up and buying condoms, then she's a slut.

If she just wants to have them on hand as birth control, that's ok.

If she has a boyfriend and doesn't have sex with other guys, then it's a good thing for her to share the cost and buy condoms. I don't care about frequency of sex; having sex with too many dudes makes a girl a slut.