Usually gut strings are quite low tension already, in my experience. Maybe she could just lower the strings, but then that will probably sacrafice volume. One interesting thing is that the bass is probably louder to the rest of the band than it is to her, so she might not have to be playing so hard to begin with. But then hearing intonation...that's another story. You don't have to be in tune to play bluegrass bass, do you?

If she is just starting to play, I think she'll get blisters regardless of what strings she uses.
Otherwise, send her over to the doublebass side of talkbass and search the strings section under "low tension". I'm sure lots of good info is to be had there.