Are you creating an account identifier?
The Account Identifier is derived from the principal ID (PID) and a subaccount number. There are conversations like this one on how to derive an account identifier using a principal.
The principal ID is the hash of the public key. If I recall correctly, you are using Internet Identity and NFID as login options. The user would then need to recover their principal ID by creating a passkey on either service and saving it. Therefore, they will then be able to access their account.