Who does best SS preamp modifications?

Better to sell what you have and buy what you want. At $2400, the Rowland Capri is very hard to beat.
State a price and ask for recommendations. There are a lot of great used products for sale on here every day.
Two names I would look at, depending on budget are Klyne and Marsh.
You never get your money back when you sell modded stuff.

If it sounded the way she likes, would she be looking to modify it? Maybe, I don't know her, but I suspect the reason behind the question has "dissatisfaction" in it somewhere.

Mod is short for modify. Modify means change. Change means make different. It does not necessarily mean make better.

Often when one looks into this course of action it is in the hope of finding a miracle or a bargain. Generally speaking, you will do far better by applying the "mod" funds to a real upgrade. Unless you have a quantifiable and well-known mod done, you will never retrieve the investment upon resale.
Sad fact.

If, on the other hand, you make a smart purchase on Audiogon, you're chances of reselling the next preamp at no loss are very good indeed.

Pick your poison. You weren't asking for this kind of advice but I hope you think about it anyway.