You already know you weren't she sounds as stroppy as her son. Maybe send a text telling her exactly what happened, her son may have beefed it up a bit once they got home. I'd personally leave her to it. If I had an issue with how somebody had treated my child I certainly wouldn't be childishly ignoring them.
YANBU. Sounds to me as if her DS gave her a different account of what happened. Some kids can be embarrassed by having to be disciplined by someone else's mum and can massively exaggerate the telling off they got. I have done it and my mates's DS has done it to me. It sounds like your response was entirely appropriate, I think you handled it pretty diplomatically. How close are you and this woman? I would give it another couple of days and - if you know her well enough - send a gentle "is everything ok?" text. If she is upset you should try to gently put your side of the story across. She may be embarrassed or awkward. Given that your dcs spend a lot of time together better to clear the air if you can...
that is really unacceptable, serious behaviour from a 10 year old. YWNU to tell him to cut it out....you should have told his mum so she could have also addressed it IMO. I would certainly have wanted to know and would have been happy to have picked him up actually and ended his day early.
I think you really should have told her what he did. I mean, he hit a child and tried to steal something, that's not just slightly less than good behaviour, that's very bad. So now I think he's told her a different version and she's annoyed with you. Also the fact that you played down what happened when she asked you probably means his version is more believable. I think you need to phone and talk to her.