The lunula is simply a reflection of the visible portion of the nail matrix.
If it is halfway down the 'nail bed' then it simply means that her bed is short and her proximal fold (eponychium) is pretty far back, thus exposing quite a bit of the matrix.
I wouldnt worry about it being square... interestingly enough... I am going to be psycic and say she has a pretty square free edge line as well.
Take care when manicuring as the lunula will be 'softer' than the rest of the plate as it is inly partially formed nail plate cells. The closer to the eponychium you get, the softer the plate will feel as the less the plate will be formed.
Beachneck has really freaking massive lunulas on his thumb. I tease him that it looks like he had a thumb/big toe transplant.
Hope this helps.