This article describes how to impersonate users when opening InfoPath forms.

If a user on Active Directory, say John, is set to have a manager, say Phil, and you have a process set up that is started by John (the originator) and needs approval by his manager (ie.Phil), you can emulate this all while logged in on one machine (eg. A VPC). In the process below you need to impersonate the user who needs to open the work item (ie. the manager).
To impersonate this user in IIS while still logged in as another user, do the following:
•  Open IIS
•  Navigate to K2V3\InfoPathService (see below).
•  Right-click and select 'Properties'
•  Click on the 'Directory Security' and click on the 'Edit' button for 'Authentication and access control'
•  Make sure that the 'Basic Authentication' is checked and enter the domain name in the 'Default domain' textbox (see below).