@chai2,
I take the opposite side, sometimes, to counter ignorance.
The man who made that decision to allow the former GF to sleep over made his decision and it was his alone. If they had sex, why would he have allowed the OP's former GF to stay so long as to ensure that there was an issue?
There is so much we all don't know but you pass out advice like you've know the people forever.
Perhaps, just perhaps, Chai, you don't know what has transpired and though your advice is good general advice, it doesn't follow that it's for all adults.
You don't know what the relationship was between that man and the former GF, or for that matter, the relationship between that man and his wife. Perhaps they had split and I recall you, just recently, telling a poster that she had no right putting any measure of control upon her ex.
Wanna try for a little consistency?