Stuck Between a Rock and a Hard Place

My Collin has this friend. I LOVE this friend's mom to death and we are good pals. We have so much in common despite our 10 year age difference (me being the younger).
Although Collin is 4, he is great with other kids and has somewhat a submissive nature. He won't fight for toys. He doesn't mind sharing (MOST of the time). But when he is around this particular friend, the sweet Colly I know disappears. He is bossy, cranky, rude and negative. I don't know what it is, but I CAN'T STAND IT! The friend is always crying and whining the entire time they are together because things aren't going the way they would like them to. Collin is always saying things like, "You can't play in my room with me." And, "Mom, I just want to read books by myself. I don't want to play." It drives me insane!!!!!
He was NEVER EVER like this before. Last summer when we met these guys, it was through a new play group that we joined when we moved to our new house. They played fine when we were at play group and it was outside. Now that it is indoors for winter, it kind of is the same thing. But I have noticed this friend pretty much cries and whines with everyone, so it's not just Collin. It may be that she is an only child???
Anyway, MY belief is that it stems from the HUGE incident last summer between the two kids. We stopped at a yard sale so I could search for photography props and the kids were playing just a few yards away in the grass. All of a sudden I hear a blood-curdling scream, so I run over to my son and he is crying harder than he ever has before and holding his face. I guess this little person had hit Collin in the face with an antique (wood and metal) tennis racket because he wouldn't let her have the ball AND the racket. 
His little face was bruising quickly and he had a scratch on his cheek and side of his head. Later on his eye blackened. I was SO mad...I don't know what I did to hold myself back from grabbing that racket and beating the shit out of that kid. (Honestly, that is just who I am. I think that way. Would I ever do it? Maybe. If they would have been older, like teenagers, and that happened, I seriously would have considered it. Jail or not...don't beat up my kid!! lol) The mom TRIED to make her kid say sorry to Collin, but to no avail. This kid was kicking, screaming and hitting saying there was no way they were going to say sorry. So mom dropped it! Wow..no wonder the kid is the way they are. They don't have to do anything they don't want to do!
So we went home and he was so so sad; not understanding why that happened to him. After that, there was no apology because this friend had told her mom it was an accident. Bullshit. But being mommy's sweetheart, the kid was believed. I was furious. I couldn't let it down for the longest time. We didn't hang out for quite a while.
Finally after realizing there was going to be no apology, I had to let it go. That really is so unlike me. I was mad at mommy for not even apologizing to me. I mean, if your kid is refusing to apologize, that's one thing but a mom should for her kid's actions.
After that, Collin was super defensive. He would squint and hide behind me when we saw her. He didn't want to play. Then, in a turn of events that apparently I am unaware of, he is super OFFENSIVE. He is excited to have someone to play with....until they get here. The minute they walk through the door, he is rude and mean. He will go to his room and play alone, leaving this friend up here crying (real tears) and whining to no end. She doesn't understand why he doesn't want to play. ???
So that leaves me in a tough spot. The kid's mom and I are like long lost sisters. We have tons in common, but enough difference that it keeps things interesting. We love to talk and craft. She is never without her kid just like I am never without mine, but Collin is so super naughty when she is around. WHAT DO I DO!?!?! HELP ME!!!!!!!!