She should check with her radiation oncologist, but if I were to bet, it's probably just a reaction from the radiation. Radiation can dry your skin out pretty badly. It also makes you extremely tired. The tiredness should ease up in a few weeks, but it may take longer than that. It's not that the radiation is difficult, it's just a long, arduous process that seems to take a lot out of the body.
For your peace of mind, see if you can accompany her to her next oncologist appt to inquire about the rash. The tiredness is common, I'm not positive about the rash. Always better to ask the doc who's doing the treatment just to make sure.