She obviously had a different view of your relationship...and I'm not talking romantic or anything....like maybe she thought you were close friends. She also probably thinks that she does good work and isn't accepting that someone else doesn't agree. The bill for past work???? I'm thinking she either did some stuff as a perk..like maybe she was never going to charge you... and then decided to bill you for it out of anger when you fired her. Hostage records is a ploy to get her money, I assume, as well as more revenge. Nice mess.
I've only ever fired one person. She had self-imposed medical problems (abuse of prescription drugs I've been told), couldn't learn the job in a normal amount of time (most have the basics within a week, she was still barely able to ring a sale by herself after 6 weeks), made inappropriate sexual jokes to the customers and other employees, and would often have to go home, usually in tears, early. The most horrible person I've ever had to deal with. My father and the manager were in a state of ...gawd, I don't know what. I guess they were afraid she'd cry at them??? I finally got to the point where I couldn't take another day with her and when she called in to say her son had missed the bus again and she'd be late (again), I told her to go ahead and stay home....forever. Awful.