Toxic family members

Okay so this is kind of an awkward thing for me, but I don’t have a lot of friends who have kids or have a similar situation so I’m just looking for advice or if you’ve have a similar situation. My MIL was in a 13 year relationship with someone and he had previously cheated, about 4 years ago, and she forgave him and we all decided to move on...my son grew very attached to him and he and my MIL were the main babysitters for our son. This past April we found out he was cheating on my MIL again with the same woman...okay so my husband and I decided to cut the guy off, clearly he is toxic and not someone who values our MIL like we thought. Recently we have found out that my MIL is still in contact with her ex and they are even on vacation together right now...my husband and I have agreed still that he is not someone we want in our or our sons life...there’s more to the story of the ex but I’m not gonna dish out all the dirty laundry, but he is not a good person in our opinion. It doesnt help that my MIL is very vocal and has always been one to air out all of their issues and especially when the second incident happened. I have felt so torn because my son loved him sooo much and has eventually stopped asking about him, but it has caused me a lot of heartbreak for my son. And I worry this will cause an issue further down the road if my MIL continues this relationship that we have chosen to not have with him. Anyway, any thoughts?! Sorry so long! But I guess I feel like I need reassurance that our decision was right and moving on without him in our lives was the correct call. It’s so hard being a parent, I just worry!