Active Membership but No User Account in WPMU Memberships

Hello,

We have a paid member who is saying he tries to activate his account with a key, but the key does not work. Usually when these types of errors appear, we just go in and delete the User account (thus deleting the Membership registration as well) and have them try to sign up again. This time, however, there is a Membership registration for the person, but no User registration, so we are unable to delete the User and allow them to retry registration. Could you please help debug this issue and delete this user so they can try again? We're not sure why some people received this keycode validation email, but it seems to have only affected a few people awhile ago, and does not affect new registrations at this time. The user at issue is Roderick and their email is caseykoby@bell.net.

Please advise at your earliest convenience. Thanks!