My dad has a Winchester model 1873, serial number is traced back to 1875 production date. 20" barrel, 44-40. He is trying to figure out if he wants to sell it, or refinish it and keep it. I know very little about lever guns so i figured i'd leave this one up to the hive mind.

It's all original with light pitting, stock is cracked but functional. even has the original saddle loop. I'd say 75% condition or so. not bad for a 137 year old rifle.

Leaving aside issues like whether the barrel is original length, etc, an 1875 model '73 rifle model in 75% original condition would start at around $12,000 to $15,000.
If it's a round barrel Carbine model value would be $14,000 to $17,000.

If you'd like to reduce that by possibly $10,000 or so, simply have it refinished, or just "clean it up" a little. Then you'll have a $5,000 rifle at best.

If you want it refinished, the only way to go is to have Doug Turnbull RESTORE it to original factory condition and finish. Turnbull is a world class firearms restoration service and cost will be VERY, very high. This will lower the value but not as much as having it simply refinished with modern bluing and wood finish.

The value of these rifles is in their original condition, even if it's rough. A refinish or a "clean up" destroys the value and leaves you with a far, far less valuable rifle.
"Clean it up" means just about anything other than applying some rust preventing lubricant in the bore and coat of protective wax on the outside metal and wood.
That means something like Johnson's Paste Wax or Renaissance Hard Carnuba Museum Wax, NOT car wax.

Here are some 1873s from a local gun shop.
They tend to have high prices, but they also have stuff you can't find just anywhere.

Don't refinish it!!