I would encourage him to buy a new belt, keep the one he has now so that when he gains all the weight back, he won't skip a beat, then you buy the skinny belt back from him at half price.
How can anyone in his right mind refuse such a deal??
P.S....my Dad was a tailor, and when guys brought in pants to have him take them in at the waist because they had lost weight, my Dad would purposely leave the access material intact so when they gained it back, he would have material to let back out...he never told the customer he was doing it, but we always saw them come back...