You need 4,000 miles to preserve 1k, your mate wants to go from IAD to LAX and back. So:
(i) you buy in his name a ticket to say Allentown (whatever flight is cheap that day) and LAX to (say) Bakersfield;
(ii) you buy in your name the ticket your mate wants, making sure the flight to LAX is on the same day as the Allentown flight and the return flight is the same day as the LAX - Bakersfield flight;
(iii) you (or your mate) check in on line for everything;
(iv) he goes to Dulles, shows his ID and his Allentown boarding pass at security. They'll be cool. Rocks over to the RCC, shows them your boarding pass (and your 1k card) has a drink, goes over to the LAX plane, shows your boarding pass, gets on board, has a very (1k) good seat (but not a UDU
);
(v) your mate no shows for the Allentown flight. United will not care. He will not get any miles for it - but who cares.
(vi) does the same at LAX on the way back;
= SUCCESS - you have kept 1k and your mate got out to see the wife/girlfriend/son/ drug dealer whatever for free.
The bummer it will not work internationally, for the serious mileage runs. They do passport control at the gate - cross referencing you picture with your boarding pass for that plane.