I'd be upset as well, but remember it's nothing unusual!
a good heart-to-heart is in order, try and find out what happened, was DS provoked or silly or nasty? was it an accident or does he just not realize his own strength?
let him explain what happened today and in general what's going on.
let him express what he feels and why, you both really need this.
hug him, tell him you love him, you are on his side, although he's got to tone it down.
you can sort it, you know him you'll find a way.
on the other hand I would have done exactly what the other mum did, best policy is to nip it in the bud! she was nice and even had put up with it before, so can not fault her wanting to discuss this!
Also if my kids did anything like this I'd definitely want to know!!!!!!
I'd try have another chat to say how bad you feel and insist on paying for damage.
it must be hard for both of you, single mums with boys tend to have a more difficult time then girls' mums, it's a fact, so you are really not alone with your problem and shouldn't feel bad!
you are trying to be both mum and dad for him and that is admirable, I take my hat off to you! (I mean it).
the thing is that boys from about the age of 7 increasingly in need of good male influence and guidance to learn how to be a man, a role model really.
if DS's father is not available (or best to avoid) think about who you could rope in to help you out, to have a "bloke" talk with DS.
Is there a reasonable male friend, uncle, sport coach, neighbour, grandfather or teacher who could talk to him and be a bit of a role model for him? or at least see it from the male point of view? give him a bit of advice?
you are only one person and we all need help, so do not feel bad in any way!!!! you do need some practical help in RL (as well as emotional support) and I really hope you can find it.
big hugs, I hope this helps
(BTW if anything I said can be seen as critical, it's not. I meant what I said from a loving point of you)