That is odd that Amy's ex bailed out of the polygraph moments before it was to start. I know before polygraphs they sit you down and let you know the questions you will be asked before you are "plugged in". At least that is what I have heard an examiner say. So maybe he said "f... this" after he heard what questions were going to be asked.
Not saying he's guilty of anything or not b/c I don't know. But let's assume for a second he's not, and he went in thinking he was only going to be asked basic questions about seeing Chad that night. But then he heard they were more accusing questions like "Did you do anything to harm Chad, etc.?" and that freaked him out. Innocent people have failed polygraphs before and guilty people have passed before so they are not definite measures of truth. So knowing this I would probably be fearful in taking a polygraph test even though I was innocent just in case the tester said I failed.
On the flip side though, refusing to take a test after you're already there seems guilty to me.