[clue] ubuntu admin login as user

[Yaverot]

Okay, I'm trying to fix a problem with my Dad's system. I got him running ubuntu.

Loging in is easy, I have my own account.
switching to root (or using sudo on individual commands) also easy.

I can:
sudo su hisusername
but then the program I want to run can't access the running copy of X.

I don't know (nor do I want to) his password.  I don't want to change his password.
I'd prefer to fix this while he isn't around.
Is there a way for me to do an admin/root login of his account that gives me graphical access?

Unfortunately everything I've tried to google tells me how to login as root by user, instead of login as user by root.