As this has happened to a very low milage, fairly new bike, that has seen only one wet extended ride, it could possibly be a warranty situation. He also indicated that it was difficult to fold/unfold at the outset.
Seems that the clamp adjustment would have little bearing on smoothness of fold, that would be more of a hinge problem. If no adjustment for that, it would likely be caused by lack of lubrication. As far as the grease attracting dirt thing is concerned, that's why you grease it 'till it bleeds and then wipe off the excess in a design like this. Considering the lack of fittings at the lube ports it might be prudent to do this on a regular basis here, and make sure there is grease filling the ports. That, and the grease at the hinge joints, are the only things keeping water out of the hinge.
PS - With Dahons, don't forget to register a new bike with them to get the lifetime frame warranty. You do need to get an invoice from a bike shop that they've inspected it to qualify. Details are on their site.