You are in a very tough spot if the boss/owner is aware of the situation but refuses to take appropriate action. I would suggest two things. They will not totally solve the problem, but at least give you some breathing room. First, I would limit replies to her emails, both in the ones you choose to reply to and the length of your response. Don't get into a back and forth and always copy your boss. Next, don't pay attention to things that dont affect you directly. You said, "She doesn't get along with coworkers and has scared off multiple clients by being inappropriate or rude to them." Those are issues for the boss to address. If he's happy that she is costing him business, he needs to man up.