Today my horse took off with someone?

[Sazzy]

How experience was the friend's boyfriend? My guess would be not very if he didn't really want to canter, beginner rides bounce around a lot, they lose balance easily and most importantly they do not have the experience or the ability to know when they're in control or not. This makes it very easy for a horse to take advantage, and particularly when you're out in a fairly large group it's asking for trouble.

No horse is 100% perfect all the time, they are living animals with brains and their own thoughts and this means you cannot guarantee anything from them. Back when I used to work for a riding school, we had some ponies there that had done their jobs for years and were about as reliable as you could get a pony - but still every now and again you would hear a surprising story that someone took a tumble off them for a behaviour that was completely out of their normal.
It doesn't mean they're bad, or they're suddenly horrid ponies - it just means it was bad luck, pony was having an off day or what ever which meant a rider wound up bumping into the ground. 99.9% of the time, they'd get back on and it'd all be fine again.