Faking pregnancy pain.

My "friend" has missed a ton of work this year cause of her pregnancy. Her doctor writes her a note EVERYTIME. She admitted to me in a text that 90% of the time, she is faking the pain she tells the doctor she has. I am starting to get upset because when she is gone, I am expected to pick up her slack. And now we're working 6 days a week which makes it even more difficult for me. I'm about ready to go to HR. She even posted on face book how "doctors will believe anything". 
Ughh. I've talked to her and told her what she is doing is wrong, but she will not talk to me anymore. 
Should I let karma get her or go to HR?
*she is getting paid for every day she has off.