[mothered]

There was a situation in our workplace where a colleague recently ordered a 4,000$ camera lens from BestBuy ...he opened it only to realize it's just pieces of carboard in the lens box that purposefully matches the package weight.

Was the box perfectly sealed when received?

it could've been a previous customer who successfully returned it

Easily established, by checking with the accounts department to see If It was previously Invoiced.

or a rogue employee

Simply cross-check who picked & packed the order.

or even a rogue courier

It's certainly a possibility.

is this honest customer really screwed over and out 4k$?

If there's no damage report raised by the carrier or It was a previous return, the claim favours the customer.