So is the problem that they think it's been abused and you think it is a fairly normal reaction for a pet shop hamster that hasn't been handled? It maybe doesn't matter which it is, as long as from now on they take things slowly as others have suggested. Or are they thinking of taking the hamster back because of his behaviour or something?
If they won't listen to any advice about taking things slowly and letting the hammy settle in and taming and so on, you could try giving them a little RSPCA book on hamsters or something. As a 'ham-warming' present lol.
We got this one when we got Charlie, which is quite good and they'll learn a lot from it, but it doesn't go into specifics like territorial behaviour and taming issues.
Care for Your Hamster (RSPCA Pet Guide): Amazon.co.uk: RSPCA: 9780008118303: Books
You could show them my little taming video if that helps!
https://www.youtube.com/watch?v=kjyEme2xcq4
You could also print this out and put it inside the book folded in half - it has some useful basics on about space, depth of substrate, wheel size etc and it's also an RSPC sheet.
Hamster behaviour - Behaviour - Hamsters - Rodents
This article is also good on aggressive behaviour, why and what to do
Why does my hamster do that? A guide to the behaviour of Syrian Hamsters. | The Hamster Fanatic Forum Blog
So you could print that out and put it inside the book folded up too. Then wrap the book up in nice paper and take it round
Although it's annoying when people won't listen and want to hear it from an authority (that is SUCH a male thing!), don't get argumentative with them or they'll reject anything you offer. Just smile and be nice and say you thought this might be helpful with their new hammy as you found it helpful (
white lie - or read it first then it's not a lie lol).