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?
I think he should lotto them after 6 months. There's a spreadsheet of buyers ... one entry per user. Use some external random site based on bitcoin, like what has been used in the past, so it's very clear who won.