how to deal with lazy coworker?

My coworker is the laziest person I've encountered in any workplace. I couldn’t care less if I don’t have to pick up the loose. Unfortunately for me, whenever she is off, I have to do one of her jobs. This job needs to be done every day, according to her, but when I was doing it for her today, I found her didn’t do it last Friday when she was in office. So I ended up doing three days’ work (Fri, Sat. and Mon.). What makes me more upset was after I diligently did her job, I received an email from her "reminding" me to do the job since it was supposed to be done every day and she was too “busy” to do it last Friday.

This was not the first time this happened. I am busy or even busier. I already work 9-10 hours a day, 5 days a week. I am still relatively new to the job, and especially recently, I have piles of job on my desk need to be done. I did her job despite my own busy schedule because I thought it had to be done.

I don’t think it’s fair for me to do her job. But I am not sure how to solve the problem.
Should I jus confront her directly, or should I tell the boss the situation? Any suggestion or previous experience is appreciated.

Thanks.