Wouldn't there be a point that he should put it in the past and trust my cousin more?
I think it's a more constructive point that 'shoulding' on anyone doesn't buy much beyond a power struggle and resentments--the opposite of trust.

If cousin is smart enough to earn her father's trust in her judgment rather than argue her way into that power struggle, she can negotiate with her father as an adult who is capable of handling herself in the outside world.

She can research negotiating techniques and adopt a sensitivity to her father's history that doesn't view the man as mistrustful of HER, but rather of her safety. So she can offer him specific behaviors such as regular check in's, the ability to 'interview' her dates--who she has prepared for the grill with an explanation of her father's history, and learning other trade-offs from her father that will help him to feel more confident.

Cousin can feel lucky that her father is allowing her to date at all, given his history, which means that he's trying. She can find ways to help him to gain more comfort in her life skills rather than view him as her adversary.