I say that warranty co is full of it. Adding AC dye is what Volvo and most others do if and when the system is low on freon and "low on freon" doesn't always mean that the evap is leaking as there's other places where freon can leak AND maybe something else was previously replaced or repaired before? There's no way (without documentation) that anyone can say that if AC dye was added, then the evap (and nothing else) could be the cause of that cars previous lack of freon.

I'd ask a lawyer for his opinion.