You can find the user’s WordPress ID by going to their user page from the WordPress dashboard. The user’s ID will be displayed in the address bar of your browser as the value of the user_id query parameter.
Once you have the user’s WordPress ID, you can update their SSO record on Discourse following the steps outlined for updating the external_id at the end of the ‘Solving require_activation SSO login issues’ section of my original post. You need to be careful with this though - especially with admin accounts.