Ugh, you poor thing. I can relate to feeling a little meek in a situation like this. Although let's get it straight here. I would be absolutely furious. Whoever said the rider probably doesn't know is right. But again, we have no idea what your boarding agreement says, Italian customs, or how they perceive business. Maybe it's a normal thing in Italy?
I know in my world it isn't ok. It's a serious liability and if that girl was hurt, you'd be at fault from a normal person's perspective. Your horse is your property. You didn't agree to lessons and you aren't' getting a discount on board for your horse being utilized in their program. I would speak to the BO ASAP.