takeiteasybuttakeit do not write or make contact with this woman.
Everyone has said not to but you are still suggesting it! If you do, you are in danger of causing a problem for your son!
If you cannot let this go, book a session with a counsellor and work out your understandable hurt and anger in a safe place.
You are not wrong to feel angry BUT potentially you will stir this up agiain if you engage in letters/contact/fake sympathy for her/anger with him/aggression/revenge or passive aggressive comments.
She broke off the friendship and so it is broken off/no more.
Choose the best school for your son and stick to it.
I personally would not tell your son anything. Giving him Thr name of a child to 'avoid' or telling him the story will only IMHO make it more likely he will become friends with the child!
If anything is ever said or done deal with it as am issue. but do not expect an issue.
My dd's school has so many year 7 classes I can't remember all the names! I go to the school about twice a year.
The kids make their own friends ds and deal with themselves. My dd is not Neuro typical but has managed to make friends and do ok.
I would seriously imagine your ex friends son would be mortified if either parent used this as a chance to dredge up a ten year old 'issue'.
Personally, you see her, I would ignore her. If anyone says do you X (I would be so gobsmacked if anyone did, life is too short), take the advice of a previous poster, "We lost touch, I knew her over a decade ago." No passive aggressive comments to make anyone interested.
But honesty speak to a counsellor, this was not your fault at all but you are now the danger as you seem to not feel able to let it go.