H.W.
J.,
Regarding being hit: I would definitely have gone straight to the mother and told her "your son hit me and I had him take a time out. I'm telling you because we all know it's never okay for a child to hit another person, especially an adult, and I thought you should know." Then, the ball is in her court as to how to proceed.
Your job is to make your own boundaries clear. Has your homeschool coop had a chance to go over ground rules and ensuring that all the adults are on the same page? I would be proactive with your friend and tell her in a 'just the facts' way "I've noticed that when I give X a direction, he is sticking out his tongue and making rude faces and noises. I want to be able to (teach,guide, whatever the reason you are working together) him and I would like to do it without his being rude to me." See what she has to say.
Were it me in a coop situation, I would simply have the child sit alone in a quieter place "You may sit here until you are ready to apologize for being rude and can follow my directions with a cooperative attitude." And then I would walk away. I would also tell that parent in advance "this is what I am going to do in the future". Kids in any instructional environment must learn to respect their teachers. Even kids who are in an unschooling environment must learn to be courteous and respect those from whom they are learning.
Your group might look into a book on classroom management called "Teach Like A Champion". So often, even in play schools, the biggest asset a teacher can have is a strong and grounded classroom management style. This will bring out the best in the kids, help them to want to learn, and to help them feel their best when they are doing their best.
I'd also say this: look at the child's overall attitude toward you. When he tries to make those connections, seize those opportunities to connect and to offer constructive feedback (instead of blanket praise) whenever possible. By this, I mean "Charles, you really listened to my directions and did that with such a cooperative attitude. Thank you so much." or "I saw you waiting very patiently for your turn with such good manners. High five!" Notice when he's able to master his impulses and does things which have been previously difficult for him successfully. It will be better than a rote "good job" and show him that you noticed his effort.
And for what it's worth, J., I ALWAYS back my son's teachers up, even if I don't 100% agree (and there have been a few times in the past I would have done things maybe a little differently--but they are in charge of that environment). It's better for me to double-check later with that adult (and get the whole story) than to have him argue with them or not mind. They have a tough job and one thing I am extremely clear about with my son is that he must listen to the adults who are in charge, whether he thinks it's 'fair' or not. He can talk to me later about it if he needs to, but he's not to argue with his teachers nor answer back to them or he loses privileges at home as well. We are careful about who we leave our son with, even at school. (I'd rather homeschool than send him to a teacher I didn't trust.)
PS: This bothered me last night, and I need to add: I agree with other posters that there is something bigger amiss if a child is hitting an adult. It sounds like you like the mom, but I have to say, I would never stay in a situation where this sort of behavior was continuing. I've left a playgroup because of the actions of one child who was being allowed to continue unsafe behaviors and their mom who was happy to let other people do her parenting for her. I do understand that kids are kids, but at 6 they should have it in their grasp to resist hitting others, especially adults. This makes me wonder what happens at home when the child hits mom and dad and what their response is. I know that in our house, if our son hit us in anger, all holy hell would break loose: there would be a firm scolding and a loss of privileges (like no legos/no stay-up time for a week).
In the future, do not discipline the child, but take him straight to mom, tell her what happened, and let her know that he may not be with your group for the rest of that day until he's apologized in writing and in person. She needs to see that *you* take it seriously or she will be enabled not to deal with it directly.