Arthur, its not when the visa was issued that's important , its when you arrived in the UK that counts.
if she arrived in the great city of Manchester

on the 8th of march 2009, the earliest she can apply for ILR is when she has been in the UK 2yrs - (upto 28 days)
so 8th march 2009 + 2yrs is 8th March 2011
8th March 2011 - (upto 28days) is 8th feb 2011
in fact she could apply on the day her visa expires (23/05/11) but i would always apply for a visa ASAP, because if there are any problems (with payment being refused by your bank, etc) it gives you time to sort them out, instead of leaving it til the last minute
so she could apply btw 8 feb 2011 and upto around 19th of may 2011 ( as the 23rd is a Monday)