Most likely written down somewhere. The seed phrase is the backup method of storing a private key to a crypto wallet. You're supposed to put it somewhere safe as a way to recover the wallet if the normal way to access it (a software app or a hardware device) fails.

Brute-forcing a full 12 or 24 word phrase would take centuries to millennia, so there's only a few possibilities:

1. They just found the full phrase written on a card in a safe somewhere, in which "deciphering" it is as simple as typing it into a fucking wallet app;
2. He was smart enough to split the phrase up and keep different parts of it in different places, so they might have had to brute-force part of it;
3. They found a hardware wallet and hacked into it to recover the phrase;
4. (exceedingly unlikely) they figured out that the random number generator he used to generate the phrase was broken and had predictable output patterns.