All she has to do is call her loan servicer and ask for a hardship forbearance--they can do it over the phone. No forms. She can ask for one month or one year. In total (per FedLoan, my servicer), you can get three years total of this economic hardship forbearance.
Ideally, your sister should have called her servicer and explained her situation before the due date, but it's too late. But she needs to get in contact with them and ask if they can help--most likely the answer will be the one month of forbearance, but maybe they can just push her due date back a few weeks.
In the future your sister needs to make sure to apply early for IBR--technically anytime you leave IBR your interest get capitalized. It's no big deal now because her interest capitalizes at the start of repayment, so if she puts her loans in forbearance now she just has one extra month's worth of interest that accumulates prior to capitalization. But if she forgets a year from now, now she has a whole year's worth of interest that will get capitalized (and wouldn't have otherwise capitalized until she left IBR--likely not until she finished residency, and possibly for some time after that if she owes enough).