How to change an email that no longer exists

If your son in law can login to Geni with the current address:
http://help.geni.com/entries/432044-How-do-I-update-the-email-addre...

If not:
http://help.geni.com/entries/515502-How-do-I-update-the-email-addre...

note: "If your son in law can login to Geni with the current address" - that means he knows what e-mail address is currently associated with the profile - NOT that he has access to it - AND he knows what password he used.

Correct, - the email is just your login-name on Geni - as long as he remembers the password he used on Geni it should still work.
When logged in he change to his new email address.

If he does not remember his password it is worse because requesting a new password goes to the inaccessible email account.

Private User - It does however look that none of your son in law's ever have set a permanent password making the profile fully claimed, so they probably have just used the invitation link he got by email as login (the fact that the profile still have a manager shows that the profile is not fully claimed by setting a permanent password).

As a curator I can help you with that, - just add an identical twin and invite him with the new email into that profile and when fully claimed I can merge them together.