“I don't know — should I continue to look the other way, or rat him out to the professor?”
You know that one of your classmates cheats on his homework. You haven't told the professor about it yet. You're trying to decide what to do, so you ask your friend for advice.
I don't know — should I continue to look the other way, or rat him out to the professor?
When you're trying to make a decision and you want advice from someone, you can use this phrase:
Ben asked me out. I don't know – should I say yes?
If someone does something wrong and you choose to ignore it, you are "looking the other way".
They're stealing money. I can't just look the other way!
"Looking the other way" is a sneaky, dishonest action so this phrase is a bit negative.
"Ratting someone out" means telling an authority about something wrong that that person has done. This phrase sounds quite negative. It's the kind of thing that a criminal would say about someone that turned him in to the police:
My next-door neighbor called the cops and ratted me out.