Ok, so long story short I agreed to part boarding a 6 year old canadian sport horse (tb/percheron) last may and he was great! never had a problem with him even took him on hacks bareback. however as it got colder and I got busy with school I told the owner (who doesn't ride so I was the only one riding him) that I was no longer going to be part boarding so a month after that she asked me if I wanted to ride him no fee just ride him I of course said yes cause he was my baby. Turns out the owner had not been out to see him for about a month so without knowing this I got there and noticed that he was biting a lot not in a mean way but he'd just grab my sleeve pull a little then let go so I assumed it was playfulness. So turns out he's a complete mess, bites constantly tries to run through his bit (which he never did in the summer) bucking rearing and everything else so I let his owner know she assured me that she would be going out and working with him but that I was also more than welcome to ride whenever I please so I went out today and he's worse than ever. couldn't even mount him he will not stand will not do anything but bite constantly (doesn't actually bite my skin just my sleeves) so from this point I'm not sure how to move forward with him. I'm so frustrated that she has done this to such a beautiful horse and I am determined to work him through it I just need some advice on where to start.