In my 40 years as a pediatrician, I only saw this once.
I was examining a child, and as I began to palpate his abdomen, his mother, seated a few feet away, started to laugh hysterically. She said that whenever she saw anyone being touched, she would feel as if she were being tickled.
I called it “Sympathetic Tickling Syndrome. “ (It is a rare form of synesthesia.)