Are you on an Exchange server at work? If so, here are the steps:

This will need to be done on his pc:

1. Select Tools and choose Options.
2. Select Delegates Tab and click Add.
3. Make sure that you are viewing the Global Address List when selecting the user who wish to grant access, click Add and then Ok.
4. Set the desired permission settings for the user to add or edit your Outlook folders. Click Ok.
5. Make sure that the specified users have been added, click Ok.

Sending on Behalf of User:

1. Create a new blank email message.
2. The delegate will need to be able to edit the From field in the email. To do this, select Options > From.
3. The From field should now be visible. Select From to select who you are sending on behalf of.
4. Select the User who wants to send on behalf of, and then Ok.