Ehm, that's MY horses mane!?

I have a pony named Robbie, and he had this gorgeous mane, it was so long and thick and it is snowy white. I think it's his best feature. Me and my family went away for a week and left him in the capable hands of our horsey friends, with the instructions that noone could ride him apart from them and the woman who owns the yard. When we got to our destination, the woman who owns the yard calls me and says "One of the riding school horses has fallen sick, and I was wondering if I could use Robbie for an own-a-pony day?" (an own-a-pony day is where children who use the riding school for lessons can have a pony for the day and they groom and clean tack etc.) I wasn't going to be using him as I was in another country, so I said of course, as long as the family looking after him wouldn't need to use him. So a little girl from the riding school was assigned Robbie, and all was fine until they were given a 'manes and tails' lesson. They were platting, thinning and doing MINOR trimming. Well, the little girl who rode Robbie didn't understand minor, and cut a *TRIANGLE* out of his mane! It is unsalvageable, I may need to hog him if I ever want him to look normal until it grows in. I don't think this is fair, seeing as I lent him to the riding school for a day, an he is a show horse. It has really angered me, especially as she just left me to find out, didn't call me and say there had been an incident. I was wondering, do you think I should complain to the owner of the yard, or just leave it?