What is a tall rig?
The mast height of a tall rig is by definitiontaller/has greater length than a standard rig.A tall rig may have a larger cross section and may have more or heavier shrouds, may have an additional spreader, etc.You could use a standard size sail with a tall rig.You most likely cannot use a sail for a tail rig on a standard length rig. The mainsail could be reefed to accomodate the shorter length. A tall rig jib, without a furling system, would most likely not be possible to use. It would seem that your friend has a standard length rig and is trying to use a tall rig mainsail with it!