- third person
Most Helpful Girl
It depends. In this very specific case, it sounds like there's a couple (individuals A and B) and a third person (C).
If A has been neglecting B, causing B to turn to C, then it's both of their faults. A for neglecting B, and B for cheating. In that situation, chances are even if C didn't exist, the relationship wouldn't last anyway.
If B just started emotionally cheating and then gives up or neglects the relationship despite A's efforts to engage B, then it's B's fault. You can lead a horse to water but you can't make it drink.
C is trying to poach B away from A. If a breakup happens, it's B and C's fault. B for not establishing proper boundaries with C and allowing emotional and/or physical cheating, and C for trying to steal someone else's partner. Once again, if A is being an attentive partner, there is only so much you can do, at the end of the day the decision to cheat is B's and B's alone, A can't control that and thus can't be at fault.
Most Helpful Guy
Why not both?