Dexter has his Thing about always wanting to Make Friends With People Who Understand, he'd be kind of a cute/weird/overeager nerd about it tbh. And Frank might have a kneejerk 'no we're not the same' at first, because he's a soldier, not a serial killer, but like...the way he talks about 'the things that fix us' and even more the way he talks about his eternal war in the comics, it does seem like kind of a similar need, even if he came by differently. And I can certainly see him sympathizing with wanting to keep his family. I think they'd have a rocky start but actually maybe settle into something like actual friends, and it'd be good for both of them to have another human connection. (And for Frank to maybe get influenced by the bit of Dexter's code that says Pick Targets The Police CAN'T get/keep, instead of just assuming all police are pointless.)