It's quite easy to cancel someone's reservation...

I just did it yesterday... (legit, the person can't travel ) Just called the travelagency and told them that 1 person couldn't travel and that they may cancel the ticket. 15 seconds later it was cancelled...

I have to admit that I had booked the tickets in first place but the person was not on the same itinerary as me since I'm on a different one.

So I can assume you can cancel just anyone's ticket without a problem.