How quickly a screenplay can change

A former student asked me to look at his new screenplay. Since it's early in the term, before I'm buried with current student work, I agreed to take a look.

I was excited from the get-go because the first four pages were top notch. Clear writing, wonderful street characters, setting up our hero, a down-and-out guy who is new to being homeless. And then everything changed. We're someplace else, different characters, and over ten slow pages go by before our potential protagonist shows up as the brother of the new focus. But it's the brother with a job we stay with, so he appears to be the protagonist -- until after twenty pages we go back to the other down-and-out guy. And all the while we are moving forward at a snail's pace with long chatty dialogue scenes. I couldn't read any more and abandoned the script on page 45 of 120. What a disappointment. Frankly, if I'm going to reject a screenplay, I'd as soon dislike it from the beginning. It almost pisses me off to get lured into a story and then have the writer abandon the very thing s/he sets up. It's like a betrayal. "A story is a promise," to quote the title of Bill Johnson's book, and beginning writers are forever breaking their word. This writer hooked me on a story and then tossed it out the window. What a disappointment.