That stinks.
I always pick up shifts for people at work, and unfortunately, everyone knows I can't say no.
However, if someone calls at 4 for me to work at 5 because they've been drinking all day and laying out in the sun, my answer is NO WAY!!!!
I hate it when the girls I work with are constantly complaining about not making enough money or not getting enough shifts--then they give the ones that they have away, because, heaven forbid, it might cut into their partying time.

Whew!!
Sorry to add my own rant to your thread