I think my ex fwb is cheating on his current gf

I’ve recently logged back into the app I met my ex on and saw he was recently active. According to the tech support team the recently active status lasts for about a week. The status disappeared today. At first I though the yellow dot meant they are completely inactive but that’s not that case. He’s been with his gf for three months now. I tried to make an excuse that maybe he accidentally opened the app but he shouldn’t have it in the first place nor be recently active. He is still matched with me on the app as well but according to the app he hasn’t viewed my profile in a very long time. The gf know of me and his history and went back and forth between liking me or seeing me as a threat. Despite I don’t like her due to how two faced and manipulative she was towards me I don’t want her to get hurt like that. I’ve been cheated on before and it’s worse then being dumped for an ex. I don’t know wether to let it, confront him on the app, or let her know.