Donbean - yes you are right, they will realise when thier children get to the age of your DS. But you must remember feeling incredibly protective of him when he was a baby, and it must be quite difficult for her to watch your 3 year old throwing a ball at/to it. Of course your DS thought he was throing it 'to' the baby, but as adults we know it's 'at' it because the baby can't catch. If YOU are not going to firmly but kindly explain to your 3 year old that throwing bals and babies don't mix, what choice does your freind have?
It doesn't have to be 'telling off' other children - I am quite happy for any of my friends to ask DS to do something differently / stop doing something / do something - I trust them - that's part of being friends. Plus, a lot of it is to do with 'refereeing' tw or more kids, so it makes sense for the parent on the spot to deal with the whole situiation, not just thier child.
If i was in the room i would have explained to my DS before the situation arose that babies couldn't catch, and not to throw the ball to him. If I had not done that, for some reason, i would have told him instantly - before the other mother did, if he was throwing the ball to the baby - partly to make the other mother secure that her little one was safe and considered, iyswim. And if i wasn't in the room, I would be more than happy if she asked him not to do it. I think an average 3 year-ols is able to understand that, too - even though theri enthusiasm may get the beter of them.
id you are feeling that peopel don't like your child, (which must be very upsetting) perhaps you should look at how mucg you patrol his behaviour hen amongst others - or your own confidence in dealing with him.
I hope there is no lasting tension with your friend - good luck.