Depending on who it is she works for is there a human resources department that she could talk too? I worked for a franchise of a BIG company and had been with them for a very long time and it took them a while to settle into the fact that I was pregnant. I had intended on working full time up until I took maternity leave but being on my feet for so many hours a day I cut my hours i think when I was only about 5 months and then cut to 4 days a week at about 32 weeks before I finished up a few weeks later. With my company they had to accommodate our medical needs and it made more sense for them to do so as then I would be capable of working the hours that I was there rather then being exhausted and not being able to do anything. I forget the wording of the law now but If she changes to part time during pregnancy due to pregnancy then they still have to hold her previous pregnancy position open in this case her full time position.
She could also contact wage line and they will let her know exactly where she stands with the company she works for, the number is in the white pages.
The other option she gets her doctor to write a medical certificate stating she cant stand for longer then x amount of hours and they will need to accomodate her as much as possible being it he law or not no company wants the headlines to be 'Company refused to lower pregnant womens hours.'

She needs to be firm with them and show them that she means business and isnt going to take their crap