If Darksidekrys did not make this clear, then he, rather than she, is the one who failed to live up to the agreement, and this story is purely and simply an example of how models are vulnerable to being ripped off by unscrupulous video producers. If he merely asked her if she was ticklish, and relied on a "yes" answer to offer her a job, then he is the one who did not know what he was doing in the casting promise, this is nobody's mistake but his own, and he is a thief if he does not pay her the entire $50.
I will say again: If he made it crystal-clear to her on the phone that the $50 was contingent on an outcome which might or might not occur, then it is a different story. If he did not do this, then he is a thief for not paying the full $50.