Not flying return portion of RT award ticket
I've booked myself on a RT SAN - BGR flight in September. My wife will fly back to from BGR to SAN with me, but will be arriving separately. I tried to get her a OW award ticket, but it was going to be 60k miles because the BGR - SAN segment is in the 30k bracket and of course US doesn't do OW awards. My entire RT ticket was 45k (20k outbound + 30k return - 5k dividend miles credit card), so paying 33% more for half the flying seemed pretty poor to me (not to mention, she only has 43k miles, so it's impossible).
Can I book her a RT ticket BGR - SAN, with the first leg being BGR -> SAN, and then she simply doesn't fly the second leg? I can get one of those for 42.5k. I can't see the harm in doing so, but then again I don't understand the logic whereby a return ticket costs 33% more than one of its legs, so I might be missing something. Thanks!