Bo proof means no accusations, especially not of the dead. There’s no point to it. Really even with proof there’s no point. Let Dear Ol Mom keep her memories as is.
She might even come back with something like “Honey, we knew about it and I said to your father, as long as you don’t bugger and as long as you stay faithful to me, what you think on your spare time is up to you. We chose not to tell you because we did not want to hurt you.”