I think if the dealer were a large mac dealer with heavy volume they might give him a set of tubes to swap (maybe with a deposit , or not) returning the defective set and submitting to mac for reimbursement/ replacement. I suspect (and i could be entirely wrong) Magnolia gives mac wide spread exposure and distribution. I think few people in the store would know much about this.
This is just my take, granted. I think you have a new buyer (who aside from having the tube light up issues seems to feel there's a possibly defective socket) I have a good handle on this and if it were me i'd bring it back and have them make it right (if it were just a defective tube which it likely is, let them change it) I don't really see there being an issue. They'll make it right and he'll be happy at the end of the day.