i cannot know for sure based on your short description, however it kinda sounds like your guy does not believe that your girl friend has much physical talent for skiing. to get a better answer from this esteemed panel of experts it would help if you could give us a bit more detail on her skiing skills, her size, her skiing goals, what type of terrain she skis, and any other factors that may have lead your fitter to come up with his choice.
the big problem is that ( and i am just guessing because you are showing ireland as your location) is that many shops do not even carry a range of flexes for women. you can buy soft, soft, or slightly softer boots. the ski boot suppliers and the retail shops are a one trick pony for women.
the simple answer is that you need flex to be able to open and close some range of motion in the ankle joint to be able to re-center your hips over your feet every time you finish a turn. also for harder snow conditions and higher speeds, stiffer boots can offer better edge grip and power transmission. however boot flex generally is looked at in a feature/benefit view. usually talking about the product like it does not need a skier to get down the hill. that is a big mistake.
that said, when you go into an average ski shop you will be told the easy to understand story that typically is generated from the supplier. that story is that weaker, lower level, lighter, less aggressive skiers need softer boots, and stronger, higher level, heavier, more aggressive skiers need stiffer boots. my opinion of the supplier/average ski shop story is very low.
it is a story based only on the product and marketing, not really taking a good look at the human being that is going to buy the boot.
so what are the important human factors that should be the impetus for the flex that you are seeking?
1. OK the obvious ones i already listed age, weight, ability (self described & realize that women under estimate and men over inflate), skiing performance goals, etc.
2. Range of motion assessment of the ankle joint. when the ankle joint is either very stiff ( low range of motion) or very flexible (hyper mobile) these body types call for generally call for stiffer boots. bummer that most shops do not carry them, especially for women. normal ankle range of motion can generally be put into any boot based on the details from point #1 above, and that fits well.
3. how the boot is fit, and the natural line up of the human angles vs the boot angles play a big part in the a skiers ability to flex or drive through the boot. so once again it is not just the boot, but the way the body fits the shape and the stance angles that determine whether the skier can flex the boot for their needs.