My 8-year old son, like many boys his age, is obsessed with video games. We try to have a rule at our house where he's allowed to play certain games that are appropriate to his age, for only a specific period. During the week, if he's done his homework and has behaved well in school, then he's allowed 30 minutes. During the week-end, it's 2 hours/day.
This has been working well for us (even if he complains at times). However, he recently made a new friend who's also into video games, but the rules at his house are obviously very different. There doesn't seem to be any limit of "screen" time, and he's also allowed to play games with a rating of M (!). We think this is unacceptable and have told the boy's parents that our son is not allowed to play certain games at their house. They are quite fine with that, but somehow our son still gets exposed to games that we don't agree with (either because the friend talks about them at school, or will show the games while his parents are not watching, or the parents think that this is a mild game, and we disagree).
So there has been a lot of fighting in our house about us being unfair, about how these games are fine and we don't understand, etc., etc. The problem is that this kid's parents are very nice people, we like them, and we really like the little boy, so we don't want to end the friendship. In general they have values that are very similar to ours; this is the only area where we disagree.
And thus my question: How can I explain to my son that we disagree with these rules without giving him the impression that we think his friend's parents are bad?