She went to the guy she drunk dialed's house.
He asked out the girlfriend of the guy, the drunk girl had drunk dialed the previous night. Because the drunk girl had told BPH that, the guy and his girlfriend weren't together anymore (probably a lie the guy had said to the drunk girl to get her to fvck him). BPH believed the drunk girl, so assumed the guy's girlfriend was single. Guy's girlfriend assures BPH, they are very much still together.
To let the gay guy she came with, know she was leaving.