The best way to resolve this would be for the owner of the key to add another User ID with the desired email address. Only the owner of the key can do this: Right click on the key, select Key Properties, and select Add New Email Address at the top of the window.
On your end, you can add a new messaging policy:
If any
Recipient is <the alias email address>
Encrypt to A list of keys
Use the Edit List button to specify that user's key.