I don't think it is a good idea to go over to your friend's just because you think he might touch you again. Being secretive about it could cause both of you to feel badly. Also, no one should touch another person, especially on the genitals, without their permission. This is not respectful. I am wondering how old your friend is. That makes a difference, too.
Instead, I encourage you to talk to one of your parents or a good adult friend that you really feel safe with about masturbation and what happened. They can probably answer any questions you might have.