Should I tell her her man is cheating?

I was talking to this dude on tinder, just a quick few messages back and forth. I was thinking of inviting him over but I wanted to see more pics of him cause he was looking a lil ugly in the pics and I swiped when I was drunk. So I told him to send me his Instagram, and he did. As I’m looking on the photos i notice the same girl in a couple of photos and once I got to the bottom of his insta I could tell it was his gf. And I was like maybe they broke up and he just didn’t delete the pics cause they were all at the bottom. So I found a pic with her tagged and the girl actually posted a photo of them like 3 days ago and they’ve been dating for like 5 years. And they just came back from vacation together for their anniversary. 😑😑😑 so I texted him back on insta “your girlfriend ____ is super pretty does she know you’re on here🥴” and then he started replying and then stopped and then a few min later he unmatched me. I have his insta and hers still so I could just sm him or her butI honestly feel so bad for her. I can tell by her insta she’s super in love with the kid and he’s over here trynna cheat. Part of me wants to DM her (anonymously) once he goes back to Cali and let her know what he was up to in TX but the other side of me feels it’s not my place to say anything to her. But I know if I was her I would want to know. Any suggestions ?? (may not be the right group)

UPDATE: I did talk to the girl and she was very sweet and understanding. Definitely cleared up my conscious cause I was feeling bad. Thank you all for the great suggestions!