Some times a user(or multiple users) can face the issue of not validating using MFA.
User can get a "Response is invalid" when entering the 6 digit pin in the final stages of the validation process.
You can try deleting all aspects of his MFA config on both his mobile device and the SN instance. You can try in multiple apps like google authenticator and the MS authenticator but still same issue might persist.
You can try below steps and see if it fixes the issue:
The issue might be with date and time settings.
1) PFA and try sync with google when using google authenticator.
2) If authenticator app is on phone
1. Go the phone Settings
2. Scroll down and click on General
3. Scroll down and click on Date & Time
4. Select Set Automatically to true.
5. Close and reopen authenticator app and try again.
You can also try uninstalling and installing the authenticator apps.