I want to read his and his baby momma’s texts

So to preface, it isn’t that I don’t trust him. I do trust him. He’s done some kinda iffy things but he’s definitely made sure to earn my trust. It’s more about the baby momma. She’s abusive (sexually coerced / manipulated him while they were together, verbally abusive, went and pressed a false charge against him with the police, but then she turns around and says she miscarried their third child after he left but wouldn’t show proof even tho she claimed to go to the hospital, asking him to drive the kids to school when they miss the bus but then starting fights in front of the house when he shows up) and she’s still trying to get back with him, like sending old photos of them and nudes. And again, I trust him, it’s more like. Because of the abusive past and knowing her intentions I want to have a second set of eyes on it, fact check the bullshit she spews at him, and be able to help figure out what’s worth saving for court because they’re still in the process of handling custody and just idk, ensuring a lack of potential intimacy and knowing that she doesn’t get to have secrets with him anymore. I just don’t know how to approach it or if I sound kind of crazy. He’s been open to me reading messages here and again and I have his phone password but I just don’t know how to make clear my reasoning or if I should just drop it.