Three people, two men and a woman, are in a romantic relationship, in which they are each romantically involved with both of the others. There is no couple which is given primary status. The relationship is at a level of commitment where “boyfriend” and “girlfriend” are generally acceptable as terms.
A breakup occurs, in which the woman and one of the men remain together, while the other man is no longer romantically involved with either.
I would say “She and her boyfriend broke up with their boyfriend”. Is there something different I should be saying?
My friends would say “[we/they] broke up with [our/their] boyfriend” depending on who is saying it, a member of the couple or someone else, treating the remaining couple as a single entity. Although IME I’ve only had one set of poly friends with the setup you describe, mostly it’s Primary Couple + (his girlfriend) + (her boyfriend/girlfriend)…
If I’m reading the OP correctly, I think it was the first and the third who broke up with their second. Although, one, two, and three are all primary and the OP says that there was no primary couple.