Value is a very difficult thing to determine. Is the watch complete? Is it all original? Does it have service records? There are lots of factors to take in when trying to determine worth. And unfortunately, the 1680 is one of the few Red letters out there that for some reason, hasn't caught on like it's other counterparts have.
$5k - $6k US is the average going rate for a 1680 Red in good, near or all original condition with the accessories. Any more than that, and it's got to be pristine, NOS, or part of a famous collection (e.g. Eric Claptoin's 1680 which sold for $9k, and Mondani's Red 1680 that sold for $29k included buyer's fees).
If your friend purchased his watch new, at 1970's prices, then he's got a watch that has given him 30 years of service with a nice return on it