You may be able to limp along if you block the 2 empty slots somehow. With them blocked, a lot more air will be forced through the heatsinks and it might be enough to keep it below 90.
Or he could use something like Vnish and use the "chip frequency settings" to lower the frequency of the bad chip to the lowest point possible, that way the rest of the chips will run at the default frequency and he won't need any hardware modifications.