M.6.
ETA** I have to say that I disagree with the other posters about telling your son that he shouldn't "talk about adults that way." Good grief!!!! HE IS 10! What ever happened to the "your momma so fat" "your momma so ugly" jokes. Don't any of you remember those? The jokes were never about a person's mother . . . My God, KIDS SAY STUPID THINGS! Who runs and tattles to their mom about something a 10 yr old said? What has the world come to that we have to start censoring what a 10 yr old said, probably in passing, to a buddy about his mom?
____
I find it somewhat ridiculous and petty that your friend would be mad over something one child said to another. He is 10 for crying out loud - most kids don't have a mouth filter until their late teens (and sometimes early 20s). I think that it is just as annoying that the 12 yr old repeated something a friend said to him as it is that the 10 yr old said anything at all.
The fact that something this small upset her says she either has a drinking problem that she is so sensitive about it, or she is one of those folks who gets upset about every little thing said to them. Both ways do not have a very good outcome for a friendship unless your friend is willing to take the high road on them.
I wouldn't make my kid apologize - he made an observation and I really doubt his intention was to be unkind or ruffle feathers. I would talk to your son about how some folks get upset about comments and other folks don't and it is hard to know who is who, even adults make that mistake all the time. Don't put him in the middle or make him feel like he did something wrong - he didn't. Your friend is the one acting like a child.
I also wouldn't apologize myself other than to say "I'm sorry that what my son said upset you - he is a child and you know that was never his intent" and move on. I think I would also ask (if she really is a good friend) WHY what my 10 year old said was so upsetting.
Good luck!