Why does one person seem to have no reflection in this restaurant mirror photo?

This is very plausibly a normal mirror/line-of-sight effect, not evidence of editing.

A mirror only shows what is visible from the camera’s viewpoint after reflection. If a shorter person is seated slightly behind a taller person who is closer to the mirror, the taller person can block the reflected view even though both people are directly visible to the camera. In this case, the daughter appears shorter, seated a bit farther back, and largely masked by Mike Pence’s body in the mirror geometry.

That’s why someone can appear in the photo but not appear distinctly in the mirror. One answer also notes there may be a tiny portion of her reflection barely visible near his head, which further supports the occlusion explanation.

So the simplest explanation is basic reflection geometry and obstruction, not Photoshop or image stitching. Similar effects absolutely do occur in real photos and videos whenever the camera position, subject spacing, mirror angle, and differences in height line up just right.