The picture looks like the image of one of the Foam Bore Cleaners slopped over onto the frame and left there for a while.

Some of them say they won't harm nickel plating, some say if you use them on a nickel plated gun, don't allow the stuff to sit on the plated surfaces for prolonged times as it may damage the nickel finish.

I understand all about the copper removing abilities of the bore cleaners and the use, or not, of under-plating copper flash coats before nickel plating.

But I think here it's a simple case of a bore cleaner , or other strong cleaner of some sort having been left on the surface of the nickel for quite some time and it left it's mark,,
Scrubbing bubbles in this case.You're looking right at a near photo image of bubbles and foam in the side of the frame.
I hardly think a fire did that.

Lot's of household spray cleaners are plenty strong and can damage gun finishes.

As all ready said,,let the owner 'simply' polish the finish up and bring it back to life.
It may be that it will be a simple job.
The question is do you want to take a $500 chance on finding out for yourself and how bad do you want a 4" M34.
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Just another thought,,

I'm also wondering if the gun wasn't home-shop Electroless Nickel Plated.
The process is not hard to master and the whole 'kit' is a Brownells call away. But you can get some very interesting looks to the final finish if you are not strictly following the rules.
(Would a 34 from this mfg era be marked as such if it was Factory Nickel plated?,,or have a 'B' marking if it left S&W as a blued pistol,,
showing my collector ignorance here. But it may be an easy quick way to see if it had been aftermarket plated)

When Electroless Nickel hit the gunsmithing market,,Lots of decent factory blued guns were simply stripped of the blue, with no further polishing to destroy the factory polish lines, shine & look.
Then Electroless plated out in the garage or in the basement.
Some turned out nice,,,some not so good.

Just like bluing, it takes some experience and patience.