The kidney problems are much much more common among male cats, as are urine crystals. The first thing that I would recommend is to take her to the vet for a check up, even if you have recently done so. Behavior changes like these should always warrant a vet visit to rule out any contributing health problems.
Honestly though I think all of the changes in her life are probably stressing her out. She has made a big trip AND lost a friend. Even if she and your other cat were not close, they still live together and were used to each other. She may be mourning his death, which is common when one pet dies. I would suggest adopting another cat, to keep her company.
Of course, you have to decide if you are ready to commit to another cat so soon after losing Luther. It does take some time but you will be saving another cat's life!