I have to agree with Amom2 - this is COMPLETELY typical. I have a now 12 year old boy, and he STILL gets in these kinds of tussles with his on and off again "friends." I think your friend is ridiculous to get involved like this and not let the boys work it out on their own. All she is doing is beginning a pattern of fighting every battle for her son, which is NEVER a good thing. I understand that it is hard to see your child hurt over anything, but they need to figure these things out on their own - it is an extremely important part of growing up and developing social skills. As the boys grow, they will form other friendships on their own, and she won't need to do it for him.
I think you are 100% right in letting them do this on their own, but if you feel like it may help to do something for your own peace of mind, you may start by asking him if he wants to start having play dates with some of his other classmates. You can do this in a way that doesn't make it seem like you don't want him playing with Luke, just say "hey buddy - I saw you talking to Joe the other day, do you want me to call his mom and see if he can come over and play sometime?" This way you are encouraging play with other kids, but not making it seem like you don't want him and Luke playing together. Good luck - and I hope Luke's mother isn't like this with him in every battle as he grows up!!