So he tries to break up with her but she says no because she knows he's breaking up with her because something is seriously wrong and all she wants is for him to talk things through so she can help him. The only problem is he can't tell her because then he'd be putting her in danger and all that and she's already been through enough trauma to last a lifetime. Plus it kinda revolved around her brother's secret, which wasn't his to tell.
So he does the only thing he can think of to make her leave him and kisses some random girl. She catches him in the act (which is exactly what he wanted ofc) and she's heartbroken so she calls things off. So now she's safe from him and he doesn't have to expose her brother.
So yeah, that's literally the only situation that I actually found cheating justified, which is something I never thought I'd say go be honest.
(P.S. The two actually do end up together, but it does take a couple tries and some time to fix what's been broken.)