It's legal. He can use whatever decoys he wants as long as they are not battery powered unless on tribal lands. Will it work, let him find out. I would probably mix some mallards in with the geese before I threw in the others. I don't typically see buffies or coots with geese. 
Right....

what H2O said. It's legal to use whatever you want. You can use seagul decoy if you want.
They make great confidence decoys.