My boyfriend can't stop seeing his ex-wife

His wife and her lover had sex on his bed. He divorced her in mid-2018 and she moved out of the house. Four months later, he and I started dating.

He is a good man. He is caring, considerate, loving and respectful towards me. When he and his wife first got divorced, his ex-wife did not visit his family. Only his children did.

In 2021, when Covid started and the lockdown happened, his ex-wife asked his family if she and the kids could stay at his family’s house for a month (for better support) during a challenging time. He told me and I was not too happy about it.

After Covid, my boyfriend’s ex-wife left his family’s home. He and I then went back to being happy as usual.

But then his ex-wife began visiting his house more often in an attempt to please his family. He allowed her to do that.

I asked him why he let her back into his family’s life, and he said that even though she and he no longer love each other, they still care for each other.

When I checked his phone, I found he had taken a lot of photos of her recently- photos of her hanging out around town, even photos of her sleeping.

I told him I could leave him alone for him to reunite with his wife and for them to take care of their children. He said he could not go back to his wife because they no longer love each other and only "care" about each other and their children.

He and I have been together for 4 years. What should I do when he and his wife keep seeing each other like that, even if it is for the sake of their children?