I've no experience with crypto but a lot with UI. Is it possible that they were trying to send 500K for 1865 and entered the numbers in the wrong inputs ? Software asked them to confirm. They saw the correct digits and pressed ok.
So they may have just forgotten to add the field that sends the money in a self-loop, and by default the account was drained by the miner fee.
This is obviously a terrible design but it can't be changed.
you can do it other ways, but it'll cost more. bitcoin's fairly freeform like that.