Moving a cabinet is not usually very hard. They could move the cabinet over (maybe a few cabinets depending on what the cabinets look like) and use a filler piece at an appropriate spot. Hard to give specifics without seeing the whole kitchen.

If moving the cabinets is not an option or at least not a desirable option (cost, etc.), consider just adding to the width of the TOP and BOTTOM TRIM board on the left cabinet along with adding shelving or a cabinet (such as for spices), painted white to the side of the cabinet itself -- shelving or cabinet that is as deep as the spacing difference. It can be open shelving or, better yet, a medicine cabinet style cabinet or something such as a spice cabinet door affixed to the side of the cabinet -- you may even be able to find those in the cabinet section of the big box building supply stores rather than need to build one.

https://s-media-cache-ak0.pinimg.com/236x/5a/85/ed/5a85ed7af96acd551487054450ec0f8a.jpg

By leaving the new cabinet/spice door a few inches less in width on the front side will the cabinet doors from looking unbalanced and, with the top even and the top trim made wider to match the trim on the other side of the window, the spice cabinet/spice door will look as if planned.

HI- Having less contrast with wall color and cabinet would help and also the window frame could be the color of the wall Also having a window dressing that covers the whole space between the cabinets.