op, I'm 5'6" and 180#, this year on a 159 banana for my all mtn and < 6" pow...and its been great for blasting around, not a park guy, like doing anything else. Its just a tad big and a 156 would probably be better but no probs with the 159.
As for GF, if she is used to 149 and an easy flex feather (beginner) why not...the easy flex and 149 might feel more stable and easier to turn than something shorter and stiffer. However its unclear how experienced and aggressive rider she is. Though there are better boards that she could progress into depending on how much she goes, her aggressiveness and how much she wants to progress. My daughter at 5'2" and 125# rides a 144 for park, 148 groomers, 155 for pow though she is aggressive and riding for 7 years.