Wide angles don't necessarily distort verticals. We're dealing with two kinds of "distortion" here, tho'. One is an optical flaw, usually barrel distortion. The very best (and even some quite affordable) wide angles have little or no perceptible barrel distortion.

The second type is more a problem of perspective than distortion. When a camera is aimed upward or downward the converging lines of a tall structure seem to be exaggerated. In the worst cases a building can appear to be falling over backward or collapsing inward. This can occur even with lenses that are well corrected for barrel or other distortions.

The common method for minimizing this effect is to avoid tilting the camera, lens and film plane up (or down, as the case may be) along the same axis (hope I'm not fouling up my terminology here and making matters worse).

The simplest method is to keep the film plane the same as the building (or whatever vertical surface, be it a wall, fence, etc.) and move the lens up (or down). Technically this is call rise and fall, tho' in common parlance the term shift is used. Strictly speaking shift refers to side-to-side movement.

There are other methods but describing them is a poor substitute for demonstration with illustrations.

I'm not familiar with the Mamiya system but chances are good that the photographer you were assisting used either a shift lens or elevated the entire camera to a point where it would not be necessary to tilt the camera in order to take in the entire scene.