My therapist said put boundaries right away and don’t let her in the house ever
This.
After marrying the guy she left me for, my ex asked to come back. She had separated from him and we were steadily spending more time together with the kids. We had gone out for lunch and the kids wanted her to come back to watch a movie. We had been separated like 3 years at this point, maybe 4. She and the kids were crying. The kids got mad at me for "making" mommy leave (I cured that by a walk to the convenience store to get candy, then they were fine).
She asked to come back within half an hour... by text! She can't be without her kids anymore and she'd been thinking about it for a while. Funny. First I'd heard of it! She was still married to a guy that didn't want to let her go either.
No more joint activities for a while, and I still minimize them... and she's not to this day invited to our home other than into the driveway to pick them up or drop them off on occasion.
Lock it down, politely, but firmly.