I'm so very sorry for your friend's losses, as well as having had such what was no doubt a terrifying experience.
Despite this tragedy, luckily it is an incident that is extremely rare, and it's very important to remember not to treat bees in general as the enemy - they are vitally important to our food chain and are alreay in seriously declining numbers.
My dad kept bees for many years, and I would help him harvest the honey. He kept and bred a gentle domestic variety. I helped him for 20+ years and was never stung.
I will never forget the time some Africanized bees, much larger than our own, attacked the hive we kept at home. My mother and I watched from a window as the "killer bees" drove our bees out of their hive and killed them all. They left our hive empty and moved on.
I'm SO very sorry your friend and her horses encountered an aggressive variety of likely non-domestic bees. I cannot imagine how frightening that must have been, and my heart goes out to her over the loss of her beloved horses.