Wow, from the posts I read ( and I didn't read them all ) you guys are hard on your friends. I will agree that the timing of her comment (and the comment itself, actually) was innappropriate and uncalled for, BUT she's human, too, just like you, your five-yr-old and everyone else posting answers. We've ALL made mistakes, said or done things we regretted, used bad judgment, put our feet in our mouths, caused problems for others, reacted poorly, called names and felt bad later about it....WE ALL HAVE DONE IT.
As her friend, talk with her about it, tell her you feel she reacted poorly but find out what's going on with her, is she under stress, is she ill, etc. You'll know from your talk whether she's taking things out on you because of YOU or because of HER.
More importantly, talk with your son (kids) about when people say mean things to them or around them...that sometimes that happens, people say things they shouldn't and we should understand they are sometimes sad inside and so that's what they do; we should understand that, try and help them if we can and, most important, pray for them. If we go around EXPECTING an apology for every bad thing done to us, we'll end up bitter, angry, unfulfilled people.
Good luck. I hope you can salvage your friendship and teach your son something about this experience as well.