I'm a current active duty nurse in the Air Force. Here are her options as I see them.
1. The Air Force has (and I believe every branch has) a nurse enlisted commissioning program; it allows you to go back to school full-time to obtain a bachelor's in nursing. She would have to serve as a medic for at least several years and apply for the program later. She would have to serve extra years as a nurse in her branch when she's done. Here's the kicker: just about every medic thinks they are going to do the same thing. These programs are very competitive, and therefore I wouldn't recommend she put her eggs in this basket.
2. She could serve as an enlisted medic for 4 years, leave the military, and use her GI bill to go back to school afterwards for nearly free. That's feasible.
3. She could use tuition assistance to take classes part time while serving as a medic. Likely she will find it difficult to do her nursing clinical rotations while on active duty, therefore I think you really could only do the pre-reqs (biology, chem, etc) this way. You probably couldn't complete the entire degree while on active duty.
4. She could pay for nursing school
with loans, finish a bachelor's of science in nursing (BSN), join the military as a nurse (and an officer), and have the military pay off her loans retroactively. This is a good option.
5. She could apply for an ROTC scholarship
, complete a BSN program, and join as a nurse. This is also a good option.