My son, who is a preteen, is a close friend to a 12-year-old girl whose mother is a single parent. On a Saturday afternoon, he was visiting them in their home. They were in her bedroom talking and listing to music, and he notices a sex toy that females use for masturbation on her dresser.
Hi
Its nice to hear that your son has a close friendship with someone and also its nice to hear that you both seem to trust them.
I would sit down with your son and explain puberty of both males and females to him, during the talks you could explain about masturbation. Explain that masturbation is normal and both boys and girls do it and it makes them feel good and helps them understand their bodies.
While you're explaining this you could say that there are adult toys that are made to help women experience pleasure and that what he saw was one of those toys and answer any questions he may have about this.
Also explain hygine while masturbating.
I hope this helps answer your questions, please feel free to ask anymore and I will answer them the best I can.