Having worked in a middle school for nine years I can tell you that this mother will not listen to you. She can't believe her son would misbehave because then she wouldn't be the perfect mother....and some mothers have to be perfect.
All children, and I repeat, all children misbehave from time to time. It is normal, natural, and perdictable. She is in total denial and there is nothing you can do about it. Unless you can get the little guy on tape doing something outrageous, then you are out of luck.
Even if you approached her with the utmost respect and tried to act like you are only concerned it will go over like a lead balloon.
I would go to her and offer to have her carpet cleaned (if he went on the carpet) then explain that he only did it because his friend told him it was fine....explain that you are in total shock as well and that he won't be over again because you can't predict what he's going to do. Then explain that you really don't think your son should play with her son because yours doesn't seem to know how to act or behave when the two are together. That you aren't sure why, maybe it is some kind of weird dynamic, but you can't control the situations and they won't be allowed to play together anymore.
Maybe she will get the hint.
PS And I know from personal experience with my oldest daughter what it is like to have your child do something extremely out of the ordinary because someone else told them too. I had a situation where my daughter was doing things her little friend told her to do even though she knew not to. At one point she licked paint off the lid of a paint can because her friend wondered what it tasted like. We went round and round for weeks about thinking before she did anything and asking me/father if something was okay if she wasn't sure. I think she has it down now, but who knows. Some kids are just followers.