As much as I have liked Subaru in the past, I cannot condone her actions. I know, I've been wanting another woman to make Chiyo burn with jealousy for a long time so she'll realize what a nice boyfriend Igarashi is, but this is not quite what I was hoping for. Subaru has obviously taken advantage of Igarashi, knowing full well that he already has a girlfriend, and is actively attempting to break them apart.
Frankly, I won't blame Chiyo if she breaks up with Igarashi when she finds out how he spent his birthday. After all, if the tables were turned, would Igarashi ever believe that some dude (like Ookuma, for instance) did not take advantage of her while she was passed out? Would he forgive her for getting drunk out of her mind and then spending the night in a Love Hotel with another man on her birthday while he was away receiving some basketball award?
It also seems that Subaru implied that Igarashi had sex with her though, from personal experience, it would have been nigh on to impossible in his completely inebriated, blacked-out state to perform any sort of dance...