Only if he doesn't know what he's doing. With the proper tools and technique, the cutting isn't the problem. Cutting into the cabinet would mean structural elements will be cut out, like stretchers (spanners) and possibly the cabinet sides. If structural parts are removed, new members should be added to maintain structural soundness and to properly support the range top.

Did you recently change the range top? The cabinets should have been built to fit it without cutting.

Nope we ordered the cabinet that's deemed a "range top" cabinet but they leave it full height so you can cut it depending on how deep the appliance is.

I'm mainly just wondering why he installed it before cutting into it.

You generally always install the cabinets with the false panels before cutting the false panels. That goes for a range top, farm sink, or oven cabinet. With the right tools, like a Fein, it's totally not a big deal at all. You can't cut and shim something to fit just the cabinet. How that cabinet fits in the whole run affects the adjustments.

Ooh, that's good to hear, Green. Thanks!