It's a microwave, non-ionizing. They're pretty much easy-bake ovens which shine a monochromatic light at a color water is very black/absorptive at (a color far redder than infrared). They cook outside-in so he'd be baking his skin well before internal injury.
It would be like thermal burns but deeper. The heating is more diffuse and deeper than traditional cooking methods so I'd imagine if you did get a burn, it would go deeper into the tissue than you might expect.
