I've came across my good buddies girlfriend on tinder. Took a screenshot, sent it to him "I don't know how to tell you, but look what I've just seen". He was confused and said he'd have a talk with her. Don't know what happened, but they are still together a year after. Not my business anyway, he is not stupid.

Once my uni coursemate sent me a screenshot of my ex on tinder and said he just wanted me to know what he encountered. I said it was ok, cause we broke up already.

The circumstances really do not matter. Just get a screenshot and send it to her in a neutral manner. Whatever happens next is none of your business, truth is the most important thing anyway.