Crikey that's a lot of work for your friend and for the environmental health dept.
I think someone from the environmental health should move in with the person complaining and stay there until enough evidence has been gathered. If there isn't room in the house perhaps a tent in the garden?
I bet that would make things move along faster. At least it would with my cooking.
my friend had to phone environmental health each and every time it happened. She also kept a diary of incidents.
My dad is friends with someone form ENVERIONMENTAIL HEALTH