So is he so out of touch with her that there was no suspicion she was messing around?
Never underestimate the power of denial.
I saw one of my best friends just get cleaned out by his girl back in college. She even stole from me when he and I lived together. The only way we found out was when I was balancing my checkbook and found a couple of checks I didn't write. She had apparently been stealing money from him for months and leading some sort of weird double life.
It happens, and I remember the look on his face when he found out. It was unreal.