I have a question for someone who knows the rules. A friend of mine and I wanted to know if it was legal for defensive players to knock down wide receivers once the quarterback has left the pocket even if he still intends to pass the ball and has not crossed the line of scrimmage or does the illegal contact rule after five yards still apply?

Depends upon the rules code used.
I'm no NFL expert, but from what I read in the NFL rulebook, contact is allowed after the QB leaves the pocket.

I will let someone else answer for NCAA rules but I believe it is very similar to the high school rule.

For National Federation (High School) rules, there is no 5-yard bump zone. Contact is not allowed once the receiver is no longer a potential blocker.

There is no 5 yard zone in NCAA rules either. Once the receiver is at the same yard line or beyond the defender, he can no longer be contacted. Whether or not the QB has left the pocket has no bearing on the call in NCAA rules.