Oh, her poor precious boy who can do no wrong, eh? She obviously can't deal with the idea of her son being less than perfect. She's only got what her son said, so he's probably painted you to be a shreeking banshee having a go at him for nothing.
I feel for you. How horrible that its been turned on you like this. Do you have a partner? Or anyone else who could go with you for support?
All you can do is take a calm approach along the line of "we need to sort this out for DS's sake". Try to come across as showing some understanding at how upset the other mom must've been to have heard what her son had done, but how you must both stop this situation.
You really need to take charge of the situation now. Be confident and strong, tell them what you want and EXPECT them to work with you to deal with it.