Do you mean that he had an affair with the ex-pupil AFTER she had left the school, or the pupil is an ex-pupil as a result of that affair?
Because if she's already left and isn't under his responsibility at the time then it is neither morally or legally wrong (well it is imoral that he has a wife and kids), however if he had an affair with a student whilst he was still teaching her then that maybe would change my oppinion of him.
I guess I'd feel that somebody who I had the upmost trust of and respect for, had gone and proven that he didn't deserve any of that as he was willing to get with a student he is responsible for.
However, teachers are human beings and they will find their pupils attractive some of the time, sometimes the age gaps can be as small as 3 or 4 years, all they have to do though, is ignore their feelings, or wait until after their pupil has left to hit on them!