And also, the letter mentions coworkers as zombies on the picture, so it was a work Halloween party from 40 years ago, meaning that at the time, the behavior was approved, actively or passively, by the exec's managers (unless the exec has been an exec their whole lives, but that's not mentioned).

I'd say to have a discussion with the exec, see what they think now, and figure out how to take the picture off Facebook if possible.