It happens all over the world, but that doesn't mean it's allowed. At my station we handle KLM, CO
and more and each airline has the policy that it is forbidden to release pax info to third parties, even if they are family members, which most of the time they are. I attended courses for DL
and they stress that point heavily.
If an agent releases that info it is against the rules and he/she can get in a lot of trouble for that.
Now I've been to AMS
recently and my brother was coming two days after me on KLM. He was a non-revver so I went to the KL
information desk to ask whether he made it onto the flight. The KLM agent wouldn't give me the information. So I showed my KLM badge and she was pretty helpful after that.
If what I did or what she did was right or not I don't know.