Thanks to everyone who offered their thoughts on my friends Pieper.

I guess I could make a general summarization:
This model in excellent condition with most original finish and no warts or sins might go for between $1800 to $2000.
One with somewhat less finish but still no problems and a nice looking gun for around $1500 to $1600.
One with around half original finish, still pretty nice with no significant mechanical problems but maybe some subtle blemishes for around $1300 to $1400.
One that is all original with no serious problems but might need some more serious TLC for around $1000 to $1200.

I would describe my friends gun as having about half the gray receiver finish; 98% barrel blue with a couple very light surface blemishes; no pits or dents in the barrels; wood looks to have had a coat of varnish or shellac applied over the original finish and not sanded; it has one hole in the toe from the missing sling swivel and a couple dings in the wood in that area; a very small crack in the wood near the toe that looks to have been glued; skeleton steel butt plate has some light surface etching; the top lever is just a little right of center; a very heavy trigger pull for each barrel; both hammers fall OK and they recock OK; Safety works fine.

Thanks again and additional comments welcome.

Jolly