Trying to be objective because i quite like the idea of Boo: from what we know of her she seemed to be the driving force behind the rather twee cafe concept - with a childishly energetic 'fun' guinea pig obsession that fb seemed to go along with quite passively although she doesn't seem particularly bothered about either cafes or guinea pigs.
And the flipside was, again, quite immature complete emotional collapse when she found out her boyfriend was cheating. Maybe undiagnosed bipolar or similar - and if so, it's likely something sooner or later would have triggered her even if FB or whoever it was hadn't slept with him. So the chances are, the level of (catholic?) guilt and culpability fb feels is out of kilter with the situation.