I think the fact he has hidden it is a worry, and he is to be blamed for that not her.
I used to chat to one of the dads at the school (our sons did a after school club together so we were often there to pick up after most other parents had gone) kids were friendly and he lived on the walk home so we often walked home together with our children chatting away, we talked about each others other half, his child and mine , his child was statemented and I was waiting for mine to be so we talked about that and other every day things. He walked my children home a few times when I was working late and we exchanged Christmas cards, He was part of the gaming lot as is DP and His gaming clan set up matches with DPs gaming clan, his PC broke and I said DP would have a look at it (as he can build/mend all things) So DP fixes the PC and I call this chap up say come get it. Next day DP gets a text from this blokes wife asking "is your girlfriend sleeping with my husband?" seems that he had kept the fact he spoke to me a secret, had "hidden" emails (all of which were very every day such as do you know club has been canceled or School play moved to xxx time.
But because he had not told her anything about the fact we spoke she got it in to her head I was a man hungry trollop!!
She sent me some very nasty texts, and I had done nothing wrong, if she was going to be pissed at anyone it should have been him for hiding something in the first place, not me.
In the end I stopped him at the school and said if he did not take her to book I would go and slap her silly. I make a point of not talking to any of the dads at the school now, even the ones who's children are very good friends of my children as I can not be bothered to deal with paranoid partners.
I think the person you need to be telling to piss off is him, he is the one hiding things.