Side chick to wife (drama)

My cousin that’s like a sister to me has been dating a guy for 9 months.

They’re both 19 and he has a kid from his last relationship. My cousin has claims she wasn’t his side chick even though she was his “ best friend” through his ex’s pregnancy and she began dating him as soon as they broke up. In the beginning of the year, he broke up with my cousin to get back with his ex for the sake of his daughter but after he got into a fight with his ex, he went running back to my cousin. This also isnt this first time it’s happened, in the past he’s cut off my cousin, “his best friend” then runs back with things get hard with his ex. Now a couple of months later, my cousin is getting married to this guy bc she claims “he’s the one” and that God wants them to be together.. this is her first boyfriend and I’m honestly worried for her. She lashes out at anyone that doesn’t support her relationship and is getting married next week.. I’m super worried she’s blinded by love and is going to get hurt.

He didn’t see my cousin for a month, and his BM claims they’re still sleeping together (his mom is a witness to her sleeping over) I’m not sure if it’s true though...

my question is, do guys like this really change?

Is this a rebound relationship?

Once a cheater, always a cheater?

Is this a disaster waiting to happen?

Opinions?

To add, when he first met my cousin he was already in a relationship and hid his ex’s pregnancy until my cousin found out through a post.