He has an RWAK100 so he's not source limited, at least in theory. What silver cable, as you recommended, is $99 and why go there if you feel a $99 cable is overpriced? Seems odd.
I find cable differences rather significant but don't find price or material the most important determining factors. I once compared some new to used se530s. They were noticably but not greatly different until the new pair had a couple hundred hours on them, then any differences became to subjective. Whether it was the BAs, cable or xover bits, I couldn't say but they moved.
Well, $99 cable is not pure silver. It's alloy of copper and silver from Beats audio. I bought it because it looked nicer and was shorter than the stock shure cable.
However, I had a chance to try cables at a store, but I didn't think that the cables were worth $300-$500 they were priced at. Nevertheless, I can say with certainty that "cables do make a difference". If pure silver cables were cheaper, I would buy them for my UERM as well.
Anyways, there really isn't much upgrade we can do to the SE535. He already has a great source, SE535 doesn't need a powerful amp, so the only thing left is the cable(assuming he has high quality music).
Personally, I would not buy a cable that is priced over $200, unless I absolutely love and adore the SE535. I would save up for a CIEM or a better iem.