Anyone for tennis?

Today, the Times carried an article about a tennis coach called Claire Lyle who had an affair with one of her pupils, a 13-year-old girl. The affair lasted a year before a complaint was lodged with the police by the girl's mother. The tennis coach was found guilty and will probably go to prison.
The part about this sorry affair that baffles me is that the mother caught the couple in flagrante delicto eleven months before she finally went to the police. Mother had gone to a party on a Sunday afternoon, but had returned early because she "didn't feel she was properly dressed". And found the coach and her daughter in bed together, doing whatever it is that ladies do in such circumstances. The daughter continued her tennis coaching after that for a further eleven months before mum blew the whistle. Doesn't this strike you as odd? My other question is: what kind of party starts at 2 pm on a Sunday afternoon? And what kind of party is it that you don't feel "properly dressed" for?
I don't condone Ms Lyle for betraying her position of trust, but I can't help feeling she's as much victim as predator in this case. And there's got to be more to mum than meets the eye.