A collectible gun is worth what a collector will pay for it. If it is refinished, typical collectors will no longer be interested in it. That is, it's no longer a collector's item, it's just a "shooter" that happens to be old. "Shooter" value is always less than collectible value, how much less depends on a lot of things.
In an extreme case, something as simple as polishing part of the gun to remove rust freckling could drop the value by several hundred dollars. For another example, if your gun were a U.S. property marked 1903 with a blued finish, refinishing it would drop the collector value by 50%.
In this particular case since the gun in question is already pretty rough, (might already be considered nothing better than a shooter--I say 'might' because I'm certainly not an expert on the particular pistol under review) refinishing might not hurt the value much. In other words, if the gun is ALREADY a shooter (no collector value) then refinishing probably won't hurt the value at all.
The only way to make sure, would be to consult with a knowledgeable collector prior to having the gun refinished. If there's something about the gun that makes it special (unusual marking or configuration) then it could still have collector value and if it does, that value will be diminished by a refinish.