The booklet is for the old graveyard round the parish kirk, which is just south of the Square. St Mary's on the Rock is a different place. More recent burials are likely to be in the new cemetery, which, as you say, is on Castle Road as you come into Ellon from Peterhead.
You could always try to find a funeral notice in the Press and Journal. With a new cemetery like that at Ellon, it is not too hard to find a gravestone, if there is one, as they tend to be erected chronologically. Look for the 1969 ones (unless her husband pre-deceased her and you would have to look for the row with inscriptions for the year he died.