your lgf knows what she wants and how to get it, within the realm of what's knowable and attainable. your lgf's dad has an attachment to a social era gone by wherein what was within reach for him at a similar age was far less than what is now within reach for his daughter. probably exponentially so. his urge to control the trajectory of her development beyond his capacity to do so compounded with his being at an innate disadvantage compared to her when navigating the turf creates an anxiety, which he chooses to deal with by... attacking her behaviour. the reports which say "x amount of time on y type of device = objective damage" are coming from a certain point of view - in fact, the same point of view that creates the negative dynamic you see.
my point of view is... we live in sci-fi times. tell your lgf's dad he better get with it and fast because imposing artificial limits on something someone enjoys doing is a great way to reduce their quality of life or force them into secrecy and eventually isolation.
put simply: he's afraid of what he's unable to understand, doesn't like feeling inferior to someone much younger than he is, and probably compulsively responds to those emotions in a contrived way that ends up with a band-aid on his insecurities and her feeling miserable, and he tells himself whatever he needs to in order to accept that outcome as a delayed positive or at least inevitable.
meanwhile, he doesn't even begin to realize how much more of value he could teach her... if only he'd embrace her as the captain of their perpetual voyage into uncharted territory.
thankfully, she has you.