You mean he shouldn't have unlocked it? Well, who knows what sort of "obstruction of justice"-tasting crap could they blame him with. I'm not so familiar with law systems over there but NOT answering a door to police isn't the smartest thing to do.
However on the other side, like he said, they should've identified themselves as police force immediately, instead of shining a light into the door's peephole. So he didn't know they were police.
Tricky situation huh?
Not tricky at all.
In the US, you have absolutely zero legal obligation to open the door for anyone.
That very specifically includes the police.
They have no right to enter your home unless you either;
-Invite them in, aka Consent - simply opening the door, even a bit could later be argued by them to be an invitation. Even just a door not locked can be argued to be an invitation - lock the door. Always.
-Exigent circumstances - If they can see, or hear someone in distress for example yelling for help.
-They have a warrant.
A classic favored police tactic, is a knock, and talk... they knock, pretend to be your friend, or doing a survey, or "proactive policing" - legally they can lie and make up any excuse whatsoever. Once they are inside, good chance they can find something arrest your for.
For anyone that thinks, "Well geez, I'm not a thief, or a murderer, what have I got to hide." You're a retarded moron. Take that insult to heart.
There are so many rules, laws, regulations, and policies, that there is literally no accounting for all of them. It's not a question of whether you are breaking the law, it's question of what law you're breaking.
Now if he backed up from the door, and did NOTHING. One of two things would have happened. They would have knocked down the door, hassled him, and still left realizing they f*cked up. OR they would have just left.
Not only that, but if they broke in, and did find anything of questionable legality, they wouldn't be able to use it in court.
I can't speak to legalities in other countries, but in the US, you do not have to open the door, and you should not open the door.
The cops are not your friends. They are not there to help you. They don't even have a legal obligation to protect you. They do not get punished in any way for losing a court case. Especially not the local/state cops.
Don't make the job of screwing you over any easier for them. Keep the door locked.
