Maybe the Freebitcoin team will also reduce all prize amounts to be smaller if indeed bitcoin is worth $ 500k and we don't know what they will do if that happens but I'm sure the Freebitcoin team already has and prepared a big plan for Bitcoin when it's in that price.
The rewards from the freeroll will adjust automatically to the movement of the Bitcoin price when it goes up or down, but if the bitcoin price reaches $ 500k then the lowest value that will be obtained from the freeroll is still 1 Satoshi because that is the lowest value in Bitcoin units, or they can also change fractions Bitcoins to µBTC.