6" is about 150 mm -- do you have a photo of what she uses today? Does she use the knife freehand, "chopping" on a board, or both?
There are a few knife shapes that might work out well and narrowing things down a bit would help.
knife tends to be narrower, like a Western paring knife. The santoku
is taller and was originally a Japanese home cook's general-purpose knife. The funayuki
was originally a general-purpose boatman's knife but the name is coming to be used to describe a knife that is tall enough to work well on a board, as well as having a tip that is nimble for fine work.
For some examples:
There isn't a "funayuki" page yet, but a few shorter examples would be: