In the first place, does he really own 50k bitcoins here? How come he created a new account just to ask this? If he is a long time holder, I guess he should know by now the right answer to his question. I am curious about his true motives of the OP.
If the really meant what he said, it doesn't matter if his account is new just to seek advice from a reputable community like btt forum.
But @OP, it is impossible to sell that off at once. The fees will be quite high that you can't imagine. Or at least sell a few and hold the rest for future use. There is a maximum limit with some crypto exchanges. Unless peer to peer transaction has been agreed upon by both parties with $50,000 bitcoin.