Symmetrically encrypt their share with the chosen password.

Not with passwords of their choice.

You could use SSS in combination with BIP to derive a set of 12 or 24-word passphrases, this might be a bit more user friendly.

It already exists: https://github.com/satoshilabs/slips/blob/master/slip-0039.m...

can you give more information please?

BIP39 is the Bitcoin standard which defines the process to generate the human readable "recovery phrase" for a Bitcoin wallet based on the private key. It's a list of 2048 pre-defiend words. Your private key is chunked into 11bit groups (with a few bits for a checksum) and then assigned the corresponding word.

You could just take the result of running their chosen passwords through the algorithm and use that as the decryption key.