I would not say he is a fool, if he got his diamonds cheap, this was a valid exit point, sellorder book was empty, price close to 100k. Well played I would say. One thing you should not forget, BTC price last year was around 50% of the actual price.
I think it's more professional if we don't call anyone involved with Diamonds a fool, even if we don't understand their decision to sell when they did. I just think name calling could be a turn off to new people entering this community.