That is a terrible situation. Well, if you really can't stand it, you have two option: come right out and tell her, or do the anonymous thing. The third option is, do nothing.
I have never had anything like this personally in my department, but a couple of years ago there was this woman on our floor in a different department, who wore the same clothes every single day. A gray calf-length spandex type skirt and a white T-shirt. With no bra. She was very well endowed so it was quite obvious. (I'm still curious how she dressed for the interview!) Anyhow, after a few weeks of working there, there were finally enough complaints that HR had to step in and talk to her about it. She got some new clothes and a bra, but didn't work in our office much longer. Just last year, there was another girl that had to have a talking to. She came to work totally smelling like cat urine.
It was pretty bad. One day it was really really awful. The girl who sat behind me went into the VP's office (gagging) to complain. The girl was sent home to bathe. I kind of felt bad for her, it must have been humiliating to be told you smell bad. But geez, how can you walk into your house and not notice an odor like that? She has several cats -- I myself have 4 cats, and cleaning the litter is an every day chore. She obviously wasn't doing that.