Have your friend try this before departure:

Put a cast from shin to foot. [Zip-up casts are available from medical supply shops. Important: have an ace bandage correctly wrapped on the ankle before donning the cast, so that if TSA asks for cast removal (due to the zipper), your lack of infirmity is hidden under the ace bandage]

Wear a pair of dress pants (get from Salvation Army thrift shop) with the casted trouser leg slit open. Wear a sport coat and tie, and be well-groomed.

Get a cane, not a crutch, also from SA thrift store.

Rehearse a couple of stories about how the injury happened: in general, the stories should involve not being as young as you used to be and/or chivalry (carrying your wife/girlfriend because the beach sand was too hot for her feet) and/or pets (ask if the agent likes cats: if yes, then "slipped on floor trying to avoid stepping on the month-old kittens"; if no, then "slipped on floor when the damn cat got underfoot"). Choice of story depends on friend's and agent's ages and genders. Helps if friend has kissed the Blarney stone

At checkin, explain the misfortune, be charming but not demanding, and ask if it would be possible to get an upgrade.

It would help if your friend has elite level in the airline.

If successfull, keep the cast on until leaving the destination airport. If not successful, go to restroom before TSA, remove cast, and ditch cast and cane

Cmon folks, the OP wants some ideas on how to get upgraded, not a discussion on ethics of bribery, TSA, etc...