The user most likely is using an Agent to forward mail. You can add this person's database to your workspace, right-mouse on the database, and select Database, Go To Agents, and disable the forwarding agent. Or you can do a switch user and become that individual and disable the agent. However, first you may want to check with your company's email policy to ensure that what you are doing is in the scope of that policy. That you have a legitimate requirement to do this. Even better, if your company policy is not to allow forwarding then relaying that information to the user to not do this and to turn off the forwarding agent should suffice.
re: forwarding address is empty. It is for our users using a forwarding agent as well, so that apparently is to be expected.