How many decibels are we talking about? How much reaches the neighbor?
Honestly? It doesn't matter.
If it were important, the neighbor would have long since measured it themselves and, based on the values, for example, sought legal action.
So what is the point if the OP now wants to go and start measuring at the neighbor's place? The latter will refuse anyway. They probably have no interest in objectifying their problem. Too loud is too loud. No medical certificate will help with that. Only when the noise source is completely gone will they be satisfied. And that is unrealistic.
hatters gonna hate
I’d save the money, those two won’t become friends anyway.