We have seen Ivy take party drugs on her own multiple times, regardless if Lena wants her share too. I'd say it's just her idea of how to have fun, that goes along with casual fucking. So it's perfectly possible she simply shares this idea and practice with Holly, because she thinks that's a way for Holly to have more fun, too. And Holly evidently seems to be onboard with it and does enjoy it.Second time through, it occurred to me maybe Ivy had given her drugs. Probably not against Holly's will, but in this route Holly has been well accustomed to submitting to Ivy's desires whether she really wants to or not. It's also completely possible, as has been said, that Holly was interested in trying the drug. But at the moment it's all very unclear. (It also lends support to BlandChili's idea that it would have been better for Lena to have been present with Ivy and Holly, so both she as a character and we as players know better what happened.)
You may as well wonder if Wade and Ian pressured Holly into taking drugs while at the beach house, what with Holly being supposedly soooo submissive and all. But i don't remember people here wringing hands over that.