How do I recover an encrypted bitcoin wallet to which I don't know the password?
Sorry for your loss.

If he used early versions of Bitcoin, he may have made an unencrypted backup of his wallet.dat file somewhere. Cracking a password may be impossible, so this might be your next best option.

Wallet encryption is a feature that was implemented in version 0.4.0 on 23rd Sep 2011, so if you do find a wallet.dat file older than that it's probably not encrypted.

Bitcoin users that are aware they are holding a substantial amount of bitcoins also are probably aware that they need to back their wallet.dat file up to a location different to their PC's hard disk in case of failure. You might find it on a DVD, thumb drive, or maybe somewhere like Google Drive.