This may or may not help depending on how technical you are, but at the core a passkey works through public key cryptography, like SSH keys work. Setting up a passkey with a site involves sending a public key to the site, which they store as being owned by your account. Then, to sign in, they use your public key to generate a challenge, you use your private key to solve the challenge, and you respond proving you have the corresponding private key.
This means you never send your private key to the site, so if their database is compromised they only get a public key, which can’t do anything (it is public, after all)
No amount of phishing can get you or your client to hand over your private key, because there is no use case where that is required. There is no possibility of MITM, since your private key is never sent anywhere in any situation.
As for getting locked out of your account, losing your passkey would be the same as losing a password. You would need to follow the site’s password reset flow, and if you can recover from a lost password, you can recover from a lost passkey.
This means you never send your private key to the site, so if their database is compromised they only get a public key, which can’t do anything (it is public, after all)
No amount of phishing can get you or your client to hand over your private key, because there is no use case where that is required. There is no possibility of MITM, since your private key is never sent anywhere in any situation.
As for getting locked out of your account, losing your passkey would be the same as losing a password. You would need to follow the site’s password reset flow, and if you can recover from a lost password, you can recover from a lost passkey.