To be fair it doesn't entirely have to be direct abuse for Rachel to take issue with Brian. She's previously taken issues with possessiveness/claiming gestures from the MC, she could very well do the same with Brian.
Think back to how he first acted way back in like, week 1 of the game, when Vicky first joined the cafe. He came up to the MC, demanded her back like she was an item, and said "stay away from her, she's MINE." If he does the same thing with Hannah, and Rachel is present or hears about it, there's no way she would ignore it. It wouldn't make her inherently Team Fletcher, but it would make her very anti-Brian.
It could be a case of the enemy of my enemy being my friend, where they agree temporarily to work together to free Hannah from Brian; I'm not sure whether it's more likely that Rachel ends up having some crucial piece of information that makes us begrudgingly accept her as part of the plan, or if she just bothers her way in to the point that it's easier to roll our eyes and let her stay than to chase her off.
Either way, I doubt Brian's relationship with Hannah will go the physical abuse route, because she's the daughter of a family head. Brian is pretty stupid, but he's not so moronic as to not realize striking her would earn the undying enmity of the Gilberts. It's possible he'd lash out at someone else in his rage - Mika seems next most likely, but I don't expect it to be her because the game seems utterly unsympathetic to her, and making her an abuse victim rather than just a gold-digging simp would change that. More likely, any rage he feels about his "relationship" with Hannah crumbling would be directed back at us, leading to a fight or to him trying something else stupid. Would be interesting if he tries to trash the cafe as an act of revenge, see how we deal with that.