I suppose it depends on the type of job, but if it's your day off and they ask you to come in, you are in no way obligated to show up. If they yell at you for refusing, then that is wrong on their part.

I am someone who can easily say no, and if I were hanging out with a friend when I received the call, I would have said I was already busy and would not be able to come in. Frankly it comes across as rude, to me, that she chose an optional shift over a friend who was over visiting. My only exception would be if she really needed the money.