It depends on what e-mail service he use.
If from his ISP, you can contact them by phone, explain the situation and have a reminder or have the password reset. Make sure that you have a bill from the ISP at hand, it may your only way of proving that your request is legitimate.
If some webmail, check if there is any reminder or reset option. You may need to answer some "Secret question" or provide a valid secondary e-mail address.
If it's a local password he created to lock down his local profile, you'll need to do some digging in his notes and hope he wrote it down somewhere.
Electro, March 2012
