A man doesn't cheat because he was fooled into it or because he didn't see it coming. He cheats because he chooses to. An act of will that he exercises at your expense...
Cheating is a decision - its not an unstoppable conclusion to social flirting.
She did not make him cheat on you. You can not make a grown man do anything. He sought her out...he flirted, he chased and he got laid. He did think about you...and he decided that you didn't matter.