yes

I'd have pretty much have done what you did, told my child to tell other to not hit him (or whatever offence is done at that time) and that if child continues to come tell me. Then if I discovered it had been a stone, i'd have gone up to the child to calmly tell child that stones must not be thrown and eyes don't grow back and how dangerous it is to throw stones. I realize you didn't know this at the time.

Not sure I'd have the guts to call the mum but what is done is done and if she acts miffed with you, gently apologize and say in the moment of panic seeing the cut by his eye, you called her. She'll understand, she's a mom too. Tell her that you didn't mean to tell her how to parent (assuming she takes it this way) and go on to say that you'd like a similar phone call if the tables were turned.

I actually gasped out loud reading that it was a stone and his eye was almost hit. That stuff scares me so much. I get so nervous when I see my kids running with sticks or playing with light sabers.

hug
jm