You try to comfort her

You go back and sit next to her.

"Hey, don't cry." you tell her.

"How can I not cry? My boyfriend is cheating on me with that French slut..." she says between sobs.

"Come on, I'm sure she was telling the truth." you reassure her.

"I don't trust her, she is lying because she wants to fuck him cehind my back."

You get an idea that could reassure her and resolve the current situation if it works.

"Why don't you tell me where his room is? I'll go and check if he is really there, and then I'll return and tell you what I find."

Isabella agrees and tells you where to find your room, oblivious to the fact that she's actually talking to you.

"Alright, I'm going now." you tell her. "Stay here, I'll be back soon."

You go back downstairs; you can now enjoy the party for a while, before going back upstairs and tell her you found her boyfriend studying in his room.