The small ring on the lower body is indeed spinning because the user overtightened the cap over and over and in time the overtightening actually tried to seperate the metal parts from the brass tube, just the opposite of when you pressed them together. To induce t his problem yourself, just grab a pen and screw the cap on real tight several times and eventually the ring will come loose.
The fix is simple, just loosen the coupler from the brass and put a drop of either ca or locktight, then re-press the part to the brass tube.
You need to tell the user to not tighten the cap so much, to which you might be met with "but the cap keeps unscrewing itself if I don't". Then you need to find the appropriate o-ring and send it to the customer to help his pen close and stay closed without the extra torque which will just loosen up the ring again.
As alfred said, this is not a kit specific problem. Since learning of this, I too started assemblying my pens with permanent locktight last year and have had no complaints since.