Well, it all depends. Is it the touring 720 from the 80's, the higher-end sister of the 520? Or is it the more recent 720 "multitrack," the low-end MTB/hybrid of the last decade or so? If it's the former, just replace the middle ring unles the whole crankset is damaged. It's the latter, the rings are probably riveted together anyway, and the whole set would need to be replaced.
Of course, it seems hard to believe that a steel chainring could be so badly damaged, or that you would want to compliment someone on the Multitrack,which is a pretty pedestrian machine. So I'm assuming that it's a touring 720. Replace that chainring! And if he still doesn't want, take it off his hands and give it a loving home. It's a great bike, it'd be a shame for it to go unridden.