White chocolate mud cake with dark chocolate ganache?
I am assuming that by mud cake, you mean a tight crumbed chocolate layer cake, almost always iced with ganache.

There are a myriad recipes on the Internet for both regular chocolate, and white chocolate variants, which span the gamut of techniques and ingredients.  While they do in fact most often call for a ganache matching the type of chocolate used in the layer, there are variations, such as strawberry frosted white chocolate mudcake, or coffee filled white chocolate mud cake.  There is even at least one recipe for white chocolate mud cake with chocolate ganache.

I think you will find two preferences lead to the relative scarcity of white chocolate mud cakes with a chocolate ganache:


Chocolate ganache may overwhelm the delicate flavor of the white chocolate layers
People who like (regular) chocolate tend to really like it, and thus tend to favor chocolate mud cake with chocolate ganache


Nonetheless, there is no other reason not to pair a white chocolate layer with a chocolate ganache if you enjoy the combination; they should work well together, although the ganache will probably be dominant.

I suggest a practice run, perhaps with cup cakes, to test whether the combination works for you before trying it out for your friend.