Tbf - I feel really sorry that this happened to him, but on the other hand, I am extremely glad that a once-locked device can never be unlocked by anyone without a purchase receipt. I'm sure it deters the theft of iPhones/iPads at least a bit if they know the device will become completely useless once locked out.
Even if they know the username and password though? A thief shouldn't know your username and password, and if I have the username and password to a device, I expect to be able to get in.
Edit: Another comment chain noted that the OP says the Apple ID was locked as well. It's possible they did know the account's password, but it looks like certain situations may require one to perform account recovery to unlock an account[1].