If he signs up for a Plat Challenge, SEA-ORD-DUB-ORD-SEA in first/business will net him a little under 16,200 qualifying points and around 19,740 redeemable miles, counting the plat bonus on both returning legs. Sounds like an easy one to me.

For only a few more dollars, he can route SEA-DFW-ORD-DUB-ORD-DFW-SEA and earn a couple thousand more q-points and around 3000 more spendable miles.