We take the safety of electricity for granted, but that little shock is actually one of the better possible endings to that story. It alerted her to the fact that there's a short in the wiring - without killing her or burning her house down.
People laugh at me for being paranoid but I buy all new electrical components for my "new" machines and I either have the potted motors re-wired or I buy new repro motors for them. Jenny at sew-classic.com has everything I need and it only takes a few minutes to replace it all.
To me, this is a very small expense to avoid a fire or life-threatening shock.
When I was a little girl, I was ironing something and I went outside for just a minute to check on the little kids. When I came back in, the iron had melted onto the ironing board, burned through the cover and set the ironing board on fire. That fast!
Electricity isn't something to mess around with.