Let’s see, there wasn’t any reason for Steven to not let them pass, so they probably didn’t need to help him to cross the river. And Rarity’s actions were explicitly stated by her to be an act of sympathy. They never negotiated anything, Rarity just went and cut off her tail to comfort him.
@Background Pony
She donated half her tail just so that Stephen Magnet would stop whining. Do you really think she would consider comforting a child suffering from cancer to be beneath her?