ok, I'm really tired, so may just be a bit dense today, but how can she put sand in with her equipment? If she buries the base of the skimmer, how will she clean it? You don't want to stir up the sand, correct? Plus the heater in there and the pumps. And topping off has got to make a mess.
And yes, I believe that a mag 18 would be too much flow through a refugium. If not for the ecosystem's sake, then definitely for your sand's sake.
How big is your tank and how big is your sump. I've got a 75g and my sump/refugium is a 20 galllon tank. I partitioned off a refugium in my sump.