Für jemanden, der es nicht selber machen muß, ist nichts unmöglich.

Anyone understand what case the “der” is in?

Nominativ, it’s a Relativsatz, and “der” is a personal pronoun who is a subject in it. “es” is in Akkusativ, standing for the object.
Literally: “For someone, who doesn’t have to do it himself, nothing is impossible”.