My Friends Kid Is Outta Control!!

I'm assuming that the children are older b/c otherwise they wouldn't be unsupervised for extended periods of time...

So, I would suggest speaking to your friend directly. Let her know that the "rules" in your house are... and then spell them out. Tell her that you value your friendship, but you are concerned that your son may start imitating the behaviors and/or retaliating and that is not okay with you. Suggest that she seek-out a parenting course (there are a lot out there if you look at your county's "family services" website) and figure out how to control this little monster before he goes to school. It won't be tolerated there either and it's going to make for a really rough road ahead.

If they are young (not yet Kindergarten), don't leave them alone for many many reasons, but in this situation you both need to be there to IMMEDIATELY and STRICTLY address the behavior. Time out may not work for this child, but it's worth a consistent try!

It would be really really helpful to know the ages of your kids. Are they toddlers? If so, then it's probably just a phase and you can try to institute the "rules of the house" so your son feels protected (i.e. "in our house, when someone pushes, that person has to go to the time out chair for 2 minutes. If you push my son, you're going to be in the time out chair.") That might change his behavior.

If he's older (4+) and he's doing this and his mom isn't responding, I think that you should probably just stop having them over. Good luck.

I once heard if you do not parent your child, someone else will. Sounds like you may be that someone else. If you want to continue the workouts and want to keep your child safe, sounds like you need to come up with a solution. In my opinion you did a great job with giving him a consequence for grabbing your son. Very sad the Mom didn't step in at that point. I think at this point you need decide if you are willing to be the parent of this child while the workouts continue or this isn't something you're willing to take on and the workouts end.

Do not allow him to come into your home. Stop the workouts or do them at her house.

what's the age of the kids?
if the child is 1-2-3 years old then he's just what is it called? well he is at that age when he doesn't listen. probably the mom doesn't know what to do with him, and only time will tell if he will improve his behavior.
if these two kids are under 6, there is no reason why they should be around unsupervised. yes, you can still have your workouts but keep an eye on them as much as you can.
if you have a good relationship with the mom ask her to help you with supervising them. tell her i am afraid THEY will hurt each other (don't blame the other kid. no need to state the obvious).
i have twins. one was always on the 'attack' mode towards the other when they were 2-3 years old. it was exhausting. it had nothing to do with their upbringing, one was just mean to the other. i never left them alone, unsupervised. and i thought she'd never outgrow it. she did, around age 5. THANK GOD, or she would have seen boot camp in her future :)
so, don;t be harsh on the friend, don't stop the friendship, but be on the lookout. chances are your son will have to learn to fend for himself, as he grows up and enters school. there are 'bullies' everywhere.