Found out he monkeybranched… started WHILE we were together - do I message the new girl to tell her?

So before we broke up, he went on a boys holiday. We broke up the week after he came back - I started the convo because I could feel he was drifting for months.
It’s been 7 weeks since then.

This week, I’ve found out he is 99% seeing this new girl. When I go into her instagram.. there’s a photo of her in the same place as him, on holiday, with him in the reflection on a motorbike.
I don’t think it was pre planned, this place is popular for people to go to. But what I know for sure.. is they spent time together, him taking her to a beach on the back of his bike.

I don’t know if she knows we were together during this time. My mind is screaming at me to message her - just so she knows but also realistically because if he has lied to her.. he down deserve to just skip into a new relationship after being a sneaky ASS..

Would you message? Or should I just ignore & let her find out the hard way in a few months time when she realizes he’s an avoidant, emotionless zombie..