Thanks to everyone! I'm really thinking about your suggestions, they are really helpful! I see that there is basically a hitting and non-hitting group here, though, so I guess I have to figure out which view I follow.
I tend to think that I'm the non-hitting sort, as this is kinda why I posted in the first place. The owner wants us to whack his horse with a riding crop if she bites, and I guess I'm just not all that comfortable about it. I tried to do this with him present, and I just felt so terrible in striking her. I guess part of me was hoping to find another way to go.
chapsi, yes, I think it is partially a case of invading her space, as she has always been a little on the dominant side, and used to nip on occasions. It's only recently that it seems to be getting out of hand.
galadriel, no, I would definitely not change this horse's form of punishment without the go-ahead from the owner! However, I was hoping to find something to try other than hitting, as I am very uncomfortable with it. I was hoping to present him with some alternatives. The difficulty lies in getting EVERYONE to follow suit. I will never know if everybody is doing the same thing, thus resulting in mixed signals for this poor horse.
Princess, no, this horse has no dislikes of me that I'm aware of! She bites at everyone, even her owner. It was never a problem until about 3 weeks ago.
Which brings me to something very interesting that murphs said....YES! We have changed her routine. I was off for several weeks and the owner found another lady to ride this mare while I was gone. When I returned, THIS is when I became aware of the biting problem. I have mentioned to the owner that perhaps this is her way of saying that she doesn't like being worked (in particular, by this other lady, maybe? I had no problems in riding her!) However, if this lady is still brave enough to ride her, I'm willing to let her take my place, and, although I like this horse, I have been turned off from riding her at all as she is, well, getting scary, and I believe that it's only a matter of time until someone gets hurt. I have left the riding up to this lady, I guess in hoping that the problem will get sorted out once the mare gets more used to be worked. (PS> I was riding her twice a week, now she's being ridden almost every day).
I was riding her as a favour to the owner, in addition to taking riding lessons. But I have since decided to leave the riding to the other lady, as I like my body parts in the condition that they are in! LOL Plus I do not want to become involved in perhaps making the situation worse, since I am a novice rider myself.
I really appreciate all the helpful advice from everyone! Please keep them coming! And a few wishes that she turns back into the lovely horse that she used to be wouldn't hurt either!
Thanks again!