He was not too knowledgeable in computers and doubt he would know why this has occurred. The type of cooler is just the stock one that came with the CPU I'm guessing. The pictures kind of show more damage then what is actually there. I did some research and found that the top piece is simply to make contact with the heatsink and protect the actual chip. I'm thinking about taking some 1500 grit wetordry dipped in a small amount of iso and sanding the metal piece down (lapping) to see if I can buff some of the roughness out. I think that will hopefully stop the thermal paste corrosion?
OK, many people even recommend polishing the CPU heat spreader before putting the heat sink fan in place anyway. You can eve usen finer sand paper than 1500 grit. I believe grits up to 8000 are available, and certainly grits up to 4000 definitely are.
The CPU die itself isn't anywhere near the size of the cover, and consequently, pits out at the edge of it, should have very little effect on cooling once the HSF is put back in place.
Assuming the color in your photos is somewhere near correct, the issue is a copper salt of some kind happening. These are very recognizable by the greenish -blue color. In fact copper's spectral burning color is very close to that. So, there is, or was, cop[per lurking in either the heat spreader or, (more likely), the cooler assy, which caused a tiny bit of galvanic corrosion due to dissimilar metals in contact with each other.
Now, the link posted by Mr. Tech is somewhat deceptive, as the first posts deal with corrosion on the PINS of the CPU (the bottom side), and not the heat spreader.
Some madman wanted to put thermal compound on the CPU pins, which would short out the CPU almost instantly.
At one time, the makers of "Arctic Silver" thermal compound, recommended assembling the HSF to the CPU by simply putting a small pea sized blob of thermal compound in the center of the heat spreader, and installing it, allowing the tension between the HSF and the top of the CPU, to spread the paste.
So, IMHO, there's no need to replace the CPU, just gently polish the top, clean it with whatever flavor of pure alcohol you have on hand, apply thermal comp[ound according to directions, install the HSF fan, and be done with it.
EDIT: I'm going to qualify that last statement just a bit by saying,
"as long as the pitting isn't clear through the heat spreader, you should be good to go".
