first off, Welcome to AquariumAdvice.com!!
How much salt really depends on the SG
you intend to reach. How did your friend mix the SW
? Are you sure it is not cloudy from the substrate? Most available salts desolve pretty readily, some better then others. Befor SW
is added to a tank it should be premixed in a container with a heater and a powerhead for 24-48hrs. This will ensure that it is fully disolved. If you are new to this hobby your next purchase should be a good book. I like "The Conscientious Marine Aquarist" by Robert Fenner.