It doesn't matter as he has already flown on it (outbound leg that required RT travel.) Had the OP not flown on it (pre-departure,) then he could recaclualte and exchange (based on current prices, not lowest possible at time of original ticketing,) but one cannot go back in time and do so. The outbound was flown, the ticket was used, under the rule that a roundtip was required. The rules don't give a "hindight" ability to go back in time and reprice (although I have seen the rate desk do such a thing in the past for errors.)

He can only exchange the return for another a) return to point of origin from the original destination, b) an open jaw from the original destination to some other point in the US, or c) an open jaw from another point near the original destination to the origin city in the US. (b and c would have to comply with the rules of the original outbound roundtrip ticket, which I can't pull without the ticketing and outbound date.)

If one could "go back in time" and reprice what would have been cheapest, then noone would ever buy full fares (if they knew of this option) and would all buy the most discounted tickets, and if they needed to make a change they would use the "go back in time and find out what would have been the cheapest way to do what I currently want" method.

There are no "mulligans" in fare rules in terms of retroactive pricing.