Kanojo no Okaa-san ni Doutei wo Ubawareru Hanashi

It has become a daily routine for the protagonist, a boy with a meek personality, to get his sexually curious childhood friend Yui to watch him sexually relieve himself. They are not yet in a romantic relationship, but one day she brings him to her house and her mom is there (who happens to witness their routine...)
This is a story where a boy gets his virginity stolen by his older childhood friend's mom, that will ultimately develop into a mother-daughter threesome affair with him.
(Source: DMM, translated)