Wow, I'd be very angry if that had happened. I can't deal with people doing things to my horses when not asked. It drives me crazy. There are instances when I think its ok, but usually not. For example, one time my trainers little black filly had its blanket twisted nearly off. The front end was buckled right, but the rest of the blanket was twisted underneath her legs. I felt she might hurt herself so I went in the stall and fixed it. Unless the horse is in danger I don't think people should ever go in other people's stalls without permission. I think petting through the bars or something is fine, if that's all they're doing. If the horse bites and it rips their fingers off, its the fault of the person sticking their fingers in, not the owner. All stables have that "you are on horse grounds and therefore realize you can get injured, we are not responsible for injuries incurred" signs in my state. Its some law that states as long as you have the sign with the gov. Code on it, then you aren't liable.
Why didn't the lady try to wash the stinky stuff out of its mane instead of cutting it? Is she insane? Lol.