What to reply to ex-husbands new girlfriend (mistress)

So my (soon to be ex-) husband has been cheating on me for a long time. I found out and kicked him out. For our daughters sake we coparent great and the other day we went to grab lunch together (the 3 of us). Today his girlfriend send me a message that pictures tell everything, she found a pic of us at the restaurant, and then screenshots of the pics. I don’t know if I’m supposed to reply?!?

I’m 100% done with him, but he keeps telling me that the second I say the word he’ll break up with her and come back to me. No thank you! But should I send her anything back? Should I tell her? I’m only getting along with him for our daughter and have expressed that a million times 🙄