Bluediamond, I feel for you. I'm a new horse owner too (well, my first horse in 20-some years so might as well be my first ever I guess). I understand your reluctance to step in, and I concur with everyone who says this girl was out of line, but what do I know. :) I also feel like an idiot half the time. Yet I am a professional who isn't used to feeling like this, but there it is. When it comes to horses, I don't know as much as other people. But maybe, just maybe, they don't know everything either. Ask a lot of people's opinions, but pick and choose which ones you follow.
I guess my point is that you reaction was understandable and your desire to check with more experienced horse people on this forum was the responsible thing to do. It does sound like you haven't quite asserted yourself as this horse's new owner and that this might be the real root of the problem. Given that, as I understand it, this horse lived at this stable before you bought it, that's not really surprising. But if you wish to remain there, you may want to assert your ownership. This can be done overtly or in a lot of little ways. Go there by yourself and spend time alone with the horse. Do not allow anyone else to handle him without your permission (except for things like feedings, blanketing if you do that, and turnout). Learn to bridle so you can ride him alone - it doesn't sound like he is a high-risk horse to ride, you should be able to do a walk/trot without a trainer present every time. Put your name and contact info on the front of his stall so everyone knows you are the owner now. In other words, you need to assert yourself as his owner and protector. Do as much as you can for him - don't doubt your abilities, if you're not sure, ask someone to show you. At a point, you just need to step into those shoes and "own" this!