Green Designs is correct; you need a better tile installer that works with marble. There also better stone pencil options than what's shown as that is more of a profile design.

Without seeing the whole window, it is difficult to tell if something like this will work. I like the second photo below with the window trimmed in wood and the tile meeting the trim around the window.

Classic White Kitchen Inspired by Something's Gotta Give · More Info

Kitchen · More Info

I like schluter it becomes almost invisible and a nice clean finish to outside corners and inside ones do not require a trim piece just good install

You have the wrong tile guy. Don’t move forward with anyone who can’t miter. Do expect to pay for someone with better skills. The average good quality time contractor charges $1500-2500 for a backsplash like you want. Maybe more if there’s more not visible.