Plus, as others have pointed out, the timeline here is very hazy. She could be exaggerating the length of time, or maybe they met and started getting close around that time, but they didn't actually start seeing each other romantically until after you two broke up.

This is the biggest thing to me. OP I understand your inclination to tell her what you perceive to be the truth, but you really need concrete evidence that they were an official couple while you were still dating before messaging this girl. If you scroll back two years on her timeline and do find obvious nods to that (I.e. the “In a Relationship” status change) then maybe send her a messaged but only if you still have texts and support that back up what you’re saying. If I were her and the dude actually was cheating on both of us, I’d for sure want to know before getting married or having kids together. But there’s just too much grey area right now to go that route IMO.