Well, you know that if you tamper with your engine, no matter how or what, you loose your warranty.... And it is not up to them to prove it did come from that modification, it's up to him to prove it didn't.
The only pieces "allowed" are Nismo S tune, and only if installed by an authorized dealer.
I think legally it is as long as it is designed specifically as a replacement part for the car, and fitted by someone fit to carry out the work, they have to honour it. eg someone fitting EBC pads themselves would not mean they can get out of a warranty claim if the caliper fails.