If this were my set of rims, my first job is to isolate the problem. So I like your idea of putting the Marathons back on without rim strips to see how they mount. If they're OK, then the problem was with the rim strips. If they're not OK, then try mounting the Kendas without the rim strips to see if the problem persists. If it does, then it's most likely a manufacturing defect with the rims. You can also try Thor's suggestion of electrical tape or something similar in place of rim strips, to see if the width of the rim strip is part of the problem.
There is an outside chance that all three tire sets are defective, but that's a long shot. I wouldn't go mounting a different new set of tires just yet.
I suppose if there is good news, it's that rims can be replaced relatively easily, whereas replacing a frame would be more involved. So we can almost guarantee that if it's a rim problem, Dahon EU will step up to the plate and fix it for you, or replace with a different new rim set. Try not to get too discouraged, it'll all be sorted out eventually. Sorry your friend has to go through all this.