Hello there, /p/!

I was thinking in buy my mother a new lens for her Nikon D800 (FX), but I'm not really god with these kind of things, so I ask for your help. Wich one would be a good accesory for her camera.

She already have a 50 fixed and one that is not FX 18-135.

I know that lens are expensive, but I can afford to a limit of ~$700