If you have a means to get in touch with her boyfriend then you could try this.

Have her meet you at a bar and tell her to bring a bikini and some shorts because afterwards you're going to a friends house to swim in their indoor pool.


Now if she agrees then call her boyfriend and tell him that she's been cheating on him and if he doesn't believe you to just go to the specified bar and to play it cool and act like it's a coincidence he's there. Then have him tell her he lost his wallet and the only place he hasn't looked is her car.

Yeah i'm evil.

Of course don't go to the bar.