To quote … someone… “Don’t look for ways to say no to your players. Find ways to say yes.”
Sure, the listed alignment for white dragons is evil. But 4e cares much less about alignment than previous editions.
So the cavalier has a white dragon mount. Cool! This can continue to be a story element where the PC gradually (or suddenly) redeems the dragon. Or it can fade into the background.
Of course, that's why I'm asking. I want to say yes, but in a way that makes sense in verse.
I think I'll go with the dragon joining the party out of self interest at first (after all the characters are obviously powerful if they can defeat a tribe of frost giants, right?), but getting eventually persuaded to their way of thinking (depending on the character's RPing of course).
Also - are you using the Dragon magazine cavalier article with the dragon mount feat / class feature? As written it gives a silver dragon but that can easily be reflavored to white.
Yes, I even mentioned it on the opening post.
Mechanically it's fine since the cavalier would be getting a silver dragon steed in the next level anyway, the player just wants to reskin it to "redeemed white dragon", and I'd like to allow it since it seems like a cool idea, so I need a reasonable explanation for it.