@Anirban,
She can wear cat’s eye of any weight between 3 to 7 carat. in left hand ring finger. And please be aware if you wear any gemstone you need to follow strict some rituals / rules. Like as it on left hand every day morning during day to day routine she has to keep near some clean place above the ground level and not on the ground. If possible she can keep it near pooja mandir. The same thing to be followed during have non-veg too.
Any gemstone should properly energiesed and weared and every time you need to be energising yourself. And after you procure the gemstone revert back here to know the best muhurtham pertaining to your mom and gemstone.( Proper muhurtham comes only ones or twice or thrice in a year)
GOD BLESS!!!
Srinivasa Temkar