The picture does not show a Chart Component (Ribbon Math - Chart Component) but rather a Prime native 2D-Plot (Ribbon Plots - Insert Plot - XY-Plot).
Unfortunately Prime's native plots are missing a lot of basic features like labels, second y-axis and ... grid lines.
Instead of adding the missing features PTC had decided to integrate a third party module which provides some of these features.
This module has its own sever drawbacks, though. Its badly integrated (Chart Component), extremely slow and awkward to handle, does not scale according to the windows zoom set (an issue when using high resolution with a small size monitor) and does not even support units (IMHO an absolute no-go for a Mathcad/Prime addon).
So your customer has two options: Either using the Chart component if it fits his needs or use one of the workarounds which were posted here in the forum a couple of times which create gridlines as an additional trace.
Some links:
https://community.ptc.com/t5/Mathcad/In-MCD-Prime-4-0-is-there-any-way-to-add-gridlines-to-plots/m-p/86492/highlight/true#M34243
https://community.ptc.com/t5/Mathcad/Grid-on-a-2D-Plot/m-p/672767/highlight/true#M190679
A third option would be to simulate grid lines by using the feature to manually add horizontal and vertical markers. As these would have to be added and formatted one by one this method would only be acceptable if a small number of lines suffices.
BTW, looking at the picture you posted I would strongly advise your customer to take advantage of a main feature of Prime and add units to his calculations.
