A "friend" of my son's, not really a friend

I feel bad! My 7 year old son got off of the bus and said that the two friends that he sits with were mean to him, told him that they were no longer his friend, and moved their seats. My son is sad, and I have that awful feeling in the pit of my stomach because I can't control a thing. Even better, I am supposed to watch this boy on Saturday for 4 hours. The boy's mother called me (knowing they had an arguement) and tried to blow it off. I told her that they need to be friends again if I'm going to watch him this weekend, I can't do that to my son. Now we wait until tomorrow to see what happens. I told DS to talk to the boy and that it would probably be fine, but to let the boy know that his feelings were hurt. My son says that he doesn't want to be friends with him again, that this boy was mean to him. Is there a way that I can put him in a bubble???????? Ok, I know the answer to that, and I knew that this would happen as he got older, but this aspect of parenting stinks.