I think you handled it really well. And were more than kind and generous in giving them a second chance.
I have had a few instances happen to my sweet son when he was three at the hands of another child in his Joy School Class (A preschool where moms take turns hosting and teaching in their home). Another boy hit/scratched him in the eye before knocking him down, and I ended up having to take him to the Emergency Room because his eye was actually scratched- a huge one right across the middle, and two small ones. It was treatable by medication, and thank heavens he was fine, but there has never been that feeling inside me of wanting to go off on the kid that did it- he was older, by about a year and a half, at the time, and his mom was apologetic, but there was nothing really, she could do. I did speak with the little boy and he was told that if he kept hurting kids, he would not be allowed back in the class.
I've also had another friend babysit my children so I could go to the dentist or something like that, and when I picked them up, they had been either neglected and been sitting in their poopy diaper, or once again, my oldest had been injured by her older son, who was at least 3 years older than mine. A scratch across the face from a toy that he hit him with. I have never left them with anyone else again, and never will. That sounds harsh, but my instances happened when I was gone, and yours while you were right there. If that other child is hurting your son right in front of you, who knows what he would do if you had your back turned.
And I'm sorry, but most children are not that aggressive unless he is being exposed to something that he shouldn't be. Like too much violent TV, or mom and dad are hurting him. I know that you liked being friends with this woman, but you have to protect your child, and I think the fact that she tried to pin this on your child just shows that they don't have much accountability in their home. If she can't even be accountable for her son's behavior, than why would her son understand what it is to be accountable for his behavior, also? I think your best bet is to just let that one go...I wish I had never left my son at either of those friend's houses. He would have one less scar on his beautiful face, and one less instance of pain that could have been prevented. I know that these things happen, and kids are tough. He doesn't remember either instance, but I'll never forget!