Tell her not to drive the car until this is resolved, or find a way to disable the ABS in the meantime. Braking issues are not something to play with when it could put lives at stake!
You'll need to pull the ABS codes (not sure how on her car) to find out what's up. I had a 97 Bravada that did the same thing, except I would have to literally STAND on the pedal to get it to stop, while fighting the ABS system. After activating I think 3 consecutive times, it system will become disabled, hence her ABS light. In my case, it was a ton of debris that had built up on the right front ABS sensor causing it to think the wheel had stopped rotating. Everytime I would hit the pedal, the system would react by activating and preventing me from stopping safely. A can of brake cleaner and half an hour of my time solved the issue, but hers could be something different, so you'll need to do some investigating.
Here's a decent link I found regarding the GM/Bosch ABS system:
http://www.babcox.com/editorial/bf/bf20136.htm