Marinette is also frequently the first person he goes to when he has a problem. He genuinely trusts her judgement. And let's not forget that Marinette forgets how to string sentences together around him so it's not even a situation of them just being really close friends. It's him actively getting through Marinette's awkwardness in order to be close to her.
I love redemption arcs for victims of abuse. I think they serve as a wonderful message. A beacon of hope. But not every abuse victim has to be redeemed. Especially in a show that has other victims of abuse who break the cycle, it's fine to have a character who doesn't. It's sad but it also serves as a message to kids.