If all you do is take the dirt and grit off it the collector value is fine.
We gave him good advice so as not to remove the patina only the grunge.
Cleaning with penetrating oil and using it to suck as much crud out of the mechanism won't hurt the vale although grinding off the rust with sandpaper or a dremel will. I have several WW1 blades and am into collecting the old stuff.

Here is a WW1 bayonet. It is spotlessly clean but still has all the "patina".

If you don't remove all the crud the piece will continue to deteriorate.
Ideally you want to preserve it perfectly in the state it is now.
Get all the dirt oil and moisture out. You would be amazed how much crud comes out of those folders when you stick some penetrating oil in the crevace and work the blade back and forth. Fill it up ,work it back and forth ,clean with a paper towel,
repeat.