I'm so sorry for the loss of your friend. I know it hurts to lose one so unexpectedly - believe me.
In answer to your question - horses kick each other with alarming regularity - but serious injuries are extremely rare. Our stallion got kicked square in the jaw by one of our mares - we heard it connect and were just sure his jaw was broken. (he deserved the kick) Daughter had the vet on the phone before I could get Scooter out of the gate. Vet just chuckled and told us to keep an eye on him for any sign of pain and he would come on out. Other than a muddy hoofprint on his jaw, he didn't have anything other than some minor swelling.
Same mare also laid him flat on the ground when she kicked his rear leg out from under him when he reared up behind her and tried to bite her (no, he wasn't trying to mount her - she wasn't in heat - she was already pregnant). Didn't panic that time, just watched to make sure he got back up - which he did - no sign of lameness at all, so he wasn't hurt. After that, though, he was a perfect gentlemen with the ladies!
Plain Old Dee, horses Dancer and Rain I believe in dragons, unicorns, good men and other mythical creatures!