How appallingly rude! (Brenda, not you, OP). I wonder if she's somewhat senile, because I can't believe a rational person would behave that way.
I'd be tempted to give cheeky, silly answers. Eg:
Brenda: Has DH ever cheated on you whilst travelling for business?
You: Oh yes, he has a wife in every city. I'm actually Wife Number 23, for the record.