Well, I, for one, thoroughly understand the other mom's position. I would feel the same way. Right down to not wanting my kids to play with yours--although I'd get over that. I WOULD NOT have gone to you with that feeling. I WOULD NOT have said you "ruined" Christmas, even if you did for that child, even temporarily because my child wouldnt know what to think. I might have told you so that you could talk to your DS and remind him to be more careful at times. I WOULD NOT expect him to be punished. Basically, I would have handled it differently, but probably had the same feelings.
We also do not know how the mother handled it with her daughter. We just know how she reacted to the OP's DH.
DS6 has a little girl in his class that has told all the kids that Santa isn't real. I asked DS what he thought. He said he believed, but over the course of the long holiday season, she presented some good arguments. I told my DS that I believe in Santa (and I do, guess I'm living a lie!

) I told him what Santa stands for. That Santa is for Christians (which this little girl is not) and for believers. I also pulled out
the book, The Polar Express-]--much more to the point about believing. When DS6 asked how Santa does it, I told him simply that not every house has a visit from Santa and Santa really has a lot of helpers. I also reminded him about the International Date Line and time zones.
A little more about the not wanting my kids to play with yours... I'd be more inclined to feel that way because I'd be wondering what else your kid might spill to mine. Not necessarily the Easter Bunny or Tooth Fairy, but about other adult things like the birds and the bees. The simple fact is most kids in the USA believe in Santa, not believing is usually for older kids and adults. I'd have to wonder what other (not just birds and bees) things your child might bring up to mine in everyday conversation I might not want them exposed to yet. But, like I said, I'd get over it.