The few sets of 18s' that I have seen for sale on facebook groups and forums have all started out way over the top price wise and then dropped and dropped until selling for around the $1000 mark for 4 and 5 wheels with tyres, I have changed mine but kept the wheels to put back on when the time comes to sell, just tell him not to count on getting too much for them unfortunately.
The BFG KO2's in 18's are under $400 if you shop around, as Paull stated ROH make some 17's fit the new VX and Sahara and are legal sizes if he is concerned.
Grant.