"She saw her boyfriend with another girl."
Translation:Chonaic sí a buachaill le cailín eile.
Going back to the previous sentence "Chonaic mé mo dheartháir ag an gcóisir," we learn that the other girl was actually his sister, who'd gone over to talk to him after finding him at the party. There had been some confusion because brother had been staying with father and sister with mother when they were separated over mother's affair with Pol.
So the girlfriend actually saw the two of them, not a new girlfriend. In her defense she wasn't pathologically jealous, just a little bit tipsy from her wine drinking after picking herself up from a bad fall in the kitchen and so this added to the confusion. Even so, her own mother, the teacher who'd argued and fought with Pol and Pol's ex wife, will advise her to dump the boy on principle.