How can I manually activate a new ProSite Registration

My activation of new blogs works fine and I am getting the activation emails perfectly. I do however have a new customer that has contacted me about seeing the following message when trying to activate their site:

“That email address has already been used. Please check your inbox for an activation email. It will become available in a couple of days if you do nothing.”.

It sounds like they are trying to re-register…so I am simply wanting to help them by activating their site manually. Is that possible? If so, how?