I'm not big on children having their own computers anymore than I am on children having TVs in their rooms, etc. I know kids need computers to do their homework on these days but I'd be right there in the room with them (den/family room) reading or working on something myself or there would be trips to the library to do homework after school.

That being said, I'd call and have a conversation with the friend's mother first, tell her what you have found and your thoughts on it and that I'd like to sit my husband and myself down with the boys and would appreciate if I could make an "appointment" for her to come over and join us and then I would have a conversation with the boys (or just my son if the friend's mom was not on board), not a "talking to" and see what he says and feels and ask what he thought the punishment should be, etc. I'd definately get the computer out of his private space, tell him it was a trust issue about using his head and that when I thought he'd shown me he was more mature I'd think about allowing it back in his room but for the rest of this school year he could just do his homework in your presence or the library and I'd make sure I understood what was happening at the friend's house with the Mom (suggest how to filter/lock the computer), trying not to ban visiting if possible and keeping in touch so the boys don't just abandon the friend's house for another like that setup.