All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
First of all, we thank you for being jealous regarding your mother's honor; this is indeed how a true believer should be; so may Allah reward you. We ask Allah to protect you, to grant you steadfastness on the Right Path, and to be pleased with you; a pleasure that would make you happy in this worldly life and the Hereafter.
You should take it easy upon yourself and not keep thinking about this matter or worry about it because what happened is not related to any Islamic ruling as far as you are concerned, especially given that this happened when you were young and you did not witness it and you do not know exactly what happened.
Therefore, there is no need to ask your mother about what happened, and you have no right to be stern with that man.
Our advice to you is to completely forget about this matter. Rather, you should preoccupy yourself with what benefits you in your religion and your worldly matters.
If you continue thinking about it, then this may lead to unpleasant consequences, especially since you are suffering a kind of Waswaas (obsessive whispering), as is clear from your previous questions.
Allah knows best.