I'd be worried if kidney stones could move into your bile duct ;)
Don't worry, you'll know if they start shifting about and causing problems. Hopefully the fatfree diet will help (I found smaller portions too), I had to cut fat out drastically and it did help though mine was all skanky and split, I imagine after it got infected, so I don't think anything would have helped much save having it out.
Definitely know what to look out for an act on it like very dark wee and pale poo (and of course the horrible pain). When my GP rang to confirm my scan showed gallstones (nice birthday present that was haha!) on the Thursday she told me to look out for those symptoms, I noted quite blase that I'd already been having them - queue hell letting loose Saturday night and Sunday ending up in hospital for a week with acute cholecystitis!
In hindsight I should have got help sooner rather than just lying pathetically on the sofa after a number of hours of a violent food poisoning like episode and by the time I got to hospital I had a temperature of over 100, very low blood pressure and heartrate of 124bmp and as I got a friend to get me in had to go via the out of hours docs then casualty then admission to emergency surgery. In hindsight, I'd recommend the getting help sooner and quicker route.
I know people who have had an attack then no problems since so hopefully it'll just go away :) How is your SIL doing now?