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.
That is possible to do. But I think that can make the exchange suspicious with his account when he deposits all of that amount, and they will watch closely and pay attention to him. They will monitor his activity on that exchange, and if he is doing something suspicious or wants to withdraw all of his money, they will ask many things before they will allow the transaction.