Is it there always? As in, from the moment she turns the laptop on to the time she shuts it off, it's always in the same spot?

If so, the only fix is to replace the display. If it's under warranty, submit a claim, otherwise the cost of getting it repaired will likely cost a good chunk of a new laptop. So she will have to decide if she wants to just live with it, pay to get it repaired, or just buy a new laptop.

Replacing the display on laptops generally isn't as difficult as it may sound, but it can be a fairly involved process. So if you aren't comfortable working with electronics at the circuit board level, I'd rule that option out.