Sounds like he wants to use LS as the central user database and use the session. To get his own apps on the screen he wants to redirect the users after LSlogin to a page outside of the LS application. The other way around is more common. This has nothing to do with support of LS.
Hint:
Learn about bridging applications. You can learn a lot about integration different app when going to a php gallery e.g. coppermine and look how they integrate which bulletin-boards, without
changing code everywhere.
Or program your own application and control Limesurvey remotely:
docs.limesurvey.org/RemoteControl