Standard nibs are neither left- nor right-handed. You can get a nib ground to a left- or right-oblique, which will make for an easier time writing, but without knowing whether the lefty in question is an under-writer or an over-writer, you would just be guessing as to which oblique would be preferable.
Over time, most nibs will naturally break into an oblique, based on the writing style of the individual, that is why fountain pens are almost never lent. For most lefties, writing with a fountain pen means using a quick-drying ink and slowing down their writing speed. Many lefties also prefer a fine tip as there is less ink applied to the paper and drying time is reduced, making smearing much less likely. Of course, a high-quality nib helps.
