Get it in writing from the guy, that the warranty is void.
Did your mom go ahead and have work done?
Call K&N for further clarification.
You (your mom) may be due a refund if charged incorrectly.
But, stay cool and on top of things. Yelling & telling the guy off, and you lose the argument and may not be able to deal with them again.... just make a voodoo doll or something later
