@Forikorder It nothing, that guy who tried to trick his someone the fiancé trust, so probably the plan could be to just trick her, and after she wakes up, make her believe they did it, maybe the fiancé sudenly appearing in a love hotel room. She did seem like she was forced to drink that ... If a engaged goes at night and drink with strangers so causal (again, the guy just asking, she was happily accepting )
this was seen multiple times in anime / series or movies, if one of the pair suspect of something unfaithful, the may ask a friend to try and force a flirt, sometimes, its more inocent then others, like in Kaguya-sama, Kaguya asked Ai to try seducing Miyuki, and the guys directly reject her, saying his hearth was already taken. Confirming Miyuki's feelings for Kaguya
The fiancé is wrong for having an affair, but at the bar incident, she didnt know that, yet she was enjoying herself and (would ) drink with a stranger chad, yet not knowing about his fiancé situation, but that didnt bother her. Her reaction the the guy's invitation kind of remove the guilt of the husband. Its a good thing they dont get engaged, she is as bad as him.
@stilgar Sure some people get off with cheating plot, forgot about that crazy side of manga, wont judge I guess (?)