Tiefling Sorcerer with Draconic bloodline?

So my son has decided to play a Tiefling Sorcerer with the Draconic bloodline class feature. Although our Encounters DM shrugged her shoulders when asked and he’s already played two sessions with him. Aren’t those choices rather antithetical?

I can’t find anything that specifically disallows it, but it sounds really edgy to me.

Does anyone know if there is a ruling on this anywhere? I won’t spoil his fun if the DM allows it, but I was surprised when she did.