Well it depends on how much risk you are willing to take and how much time you want to spend.
It is fairly easy to move and sell diamonds if you are willing to take below wholesale cost. If they are in settings - all the diamonds would be removed. Depending on how big they are and the like - if they have laser serial numbers - would make a big difference in how easy they are to sell.
Take them to NYC or another city with a diamond district and you wouldn’t have too much problem moving the smaller stones for cash. In theory anything over $10k would need to be paid by check (or have to fill out one of those forms).
Jewelry is marked up usually 3x or more - so you aren’t going to be getting 53 million. If the person finds a corrupt jeweler it wouldn’t be that hard to move $100k here and there.
Larger stones that might be famous or something might have to be recut.