I’ve had a couple of dishwashers recently where the main door seal, that fits into a channel on three sides of the cabinet, has been put on the wrong way round i.e. with the ‘fin’, for want of a better word, not touching the door it’s supposed to seal. It’s difficult to describe, but all looks fine, but won’t seal against the door. Take it off turn it round and problem solved. Worth checking if you haven’t done so already. Sounds like your customer could well have been checking that and put it back the wrong way round.
Graham