Using lefthand friction shifter as righthand for rear derailleur, feasible or not?
 

I already have installed a lefthand friction shifter on one bike, if this left shifter has enough cable travel length for the righthand driving rear derailleur, I will simply use this left shifter for the rear derailleur.
If the travel length is equal as literally seen from the picture, is there any other possible problems of using left as right? I just would like to hear what you say before going to remove it for a try on another bike.

Will it bolt up onto the handlebar facing the right way? I'm thinking it might work but be goofy.

The main problem I see is that you would either have to pull instead of push the lever or mount it on the bottom of the bar.

Not a problem as long as your cables are long enough.

Some cheap Chinese fat bikes comes stock with left ratchet shifter on the right shifting the rd.

Pure friction shifters? should work, unless you have SRAM stuff

Only problem I can think of
is that it will get you used to weird habits that may be potentially annoying to unlearn should to ever ride a normal bike in the future.

do you have some Compelling Reason to set up the controls reversed?

-'I was only able to find a left handed shifter when I was dumpster diving' is Not a compelling reason.

I have done it. It works. As stated by others you get an odd cable exit if you mount t the left on the right. In my case it didn't matter since it was those long swooping beach cruiser bars.

In my case it was a cheap rusty single speed trike conversion to 18 speeds. Only thumb shifter I had in the parts bin worth the low end project. Subsequent ly replaced though

You can do all kinds of things with controls to make them work for you. I knew a guy with a bum left hand so rigged his upright bar with two DC 135 brake levers staggered and overlapping on the right side (not optimal but it worked), plus a barcon in the right end of the bar and a thumbshifter, so all his shifting and braking was done with the right hand.

Way too late to help your friend but for future reference there are brake levers available that pull two cables and can be used to operate both front and rear brakes with one hand. Here is one source:

https://www.ebay.com/p/Sunlite-Alloy...d=381482044672

I expect it is a bit of a balancing act to get both brakes to operate well but for riders with only one useable hand they do work.

Mafac made drop bar double-cabled levers for tandems and early MTB tandems sometimes had fabbed double levers for straight bars. Nice to know such specialty items are being made.