The original design was not meant to be used in an EMI polluted environment. The design worked perfectly for years until some people started to use it in environments where there are nearby power-relays which switch high DC currents.

Then just say "outside the specs, no can do" and follow with "if you want me to design a similar board for industrial apps, I will be happy to do so".

Fighting EMI problems on a board that was not designed to withstand EMI (e.g. 4 layers) is a losing battle - been there, done that (when I was young and foolish, now I am not young any more).

Erik