Sigh. So much more complicated when we like the parents, isn't it?
My son has a couple buddies he gets into mischief with. I really like the kids, the parents are nice, and yet I've had to tell the teacher directly "so and so is a great kid, and when Kiddo gets together with them, BOY! do they get into some trouble. Could you please seat them separately?"
I'm not scapegoating any of the other kids, I'm just asserting what my son needs in that moment (different peers around) to be able to do well in class.
If your child and their child are 'ganging up', separation is important. The parents want to teach their child not to rely on his buddy to misbehave with him and to learn how to get along with other kids. Likewise, when my son and a girlfriend's child started getting into mischief at both of our houses, we both were very stern with the children and mutually agreed that the kids needed to cool off on playdates for a while. We could have blamed one or the other, but recognized both children needed help.
I'd also say that your style may be more relaxed than your friends. Personally, at that young age, I would be redirecting 'ganging up on' behaviors. When my son was acting out in class or such, I never 'wrote it off' but backed up the teacher by addressing expectations at home as well. If the teacher said that she'd had a hard time with him, there were no playdates or privileges for that afternoon. My son needed to know that even if I wasn't present at the time, I did not dismiss his behavior. Teachers have a very hard job and need our support. Perhaps this dismissive attitude is concerning to your friend. I mean, I don't get all worked up about misbehavior, but I think just shrugging your shoulders and saying 'boys will be boys' is a cop out. They need to learn how to be appropriate in the classroom and on the playground.
And I can guarantee you, I'm sure the parents of the kids who are being 'ganged up on' are frustrated. If you dismiss the behavior, they may feel that you aren't up to the task of hosting playtimes and keeping their kids safe. We had to end playtimes with a couple of kids for that reason-- the parents were checked out when they hosted, or the kid just doesn't listen to adults. No one wants to host a child who isn't theirs who is a lot of work.
Please think about your attitude and your part in all of this. If you just let problem behavior go, then parents will look to you as THE problem. The parent sets the expectation. There's really no way around that reality. It's one thing for a parent to say "My child has this or that challenge and we are addressing it" and then that parent keeps an eye on their kid-- quite another to expect that anyone else who has a problem with it or needs to make a boundary-- that there's something wrong with *them*.