The extra 19 aren't perk units, the retail 1.99 BTC price includes a 6 month warranty as per above. I wouldn't expect to get back used equipment after exercising a warranty return. If he is going to mine with the groups warranty units and keep the profits in addition to his original payment great, but it would be in good form to throw something back to the group.
Also, if a unit is defective, doesn't Arklan return them to friedcat for a replacement? Or is that only after the 19 units are exhausted?
Units are supposed to be pulled from the extras for replacement so that friedcat doesnt have to deal with it. basically to issue new unit and throw away bad unit.