What do you think about using someone's hulu account?
What do you want to use it for?
I just want to stream a show
Have you asked this person yet?  If not, you need to.  You shouldn't use things that aren't yours without asking first or you're liable to get into trouble with them.
I am going to stream a show without their permission. Since they are not using it right now, I'm going to do it anyways.
Whether they are or aren't using the account is irrelevant. How would you like it if someone used your things without asking you first? It's wrong.