I've had some positive results with getting a friend going faster by having him follow my line. He was very capable, but had a couple of nasty edge catches that left him pretty bruised up and killed his confidence.
What you have to do is go on a blue, go just a little faster than their comfortable speed, make turns that you know they can make, do the turns far apart on a wide open trail that's well groomed, and most importantly, don't get far ahead. Tell them to stay close, which will end up being not so close, definitely not more than 1 turn behind. You make a turn, they should be starting their turn as you are starting your straight line edge leaving the same turn.
Do it on a steep enough blue that you can hold a little speed without linking turns back to back. In other words, go to the trees on toe side, turn to heel, ride heel edge for a few seconds until your gf can make her turn and get back on your line, then make another turn back to toe side, ride toe edg a few seconds, etc.