First, a caveat about the following advice - I am a fairly new golfer, but I've done sport-specific training for 25+ years (it just happened to be for other sports.)
I spent a lot of time considering what training aid would best help my game over the long haul, and I ended up with a heavy shafted club.
I purchased the Momentus - it seemed awful expensive for a piece of coated rebar with a grip and a clubhead, but it wasn't quite expensive enough to justify making something myself. I presume their marketing people spent a lot of time finding that exact price spot. Anyway, it's done a great job of allowing me to strengthen the specific muscles needed for golf while not creating bad swing habits (it's a lot harder to move a weighted shaft off-plane than it is to whip around a regular shaft.) As long as one practices fairly equally swinging in both directions (which is a good reason to buy the version that has a standard grip instead of the molded grip) then there shouldn't be an anatomical imbalance issue.
I swing it while standing on homemade wobble boards to improve my core strength and balance. Other core-strength exercises (think swiss-ball) would probably also benefit your GFs swing speed, which is what I suspect is the root cause of the distance issues in the longer irons.
The good news is that there shouldn't be any negative effect on her accuracy, so it seems as though the improvement would be linear and consistent.
