Some solutions.
He installes a virtual version of windows on his mac, installs warband with his cd key (or over steam acc, then his cd key automatically applies).
He uses a windows pc/laptop to log in.
You quickly do that for him on your pc (however you both should trust each other then and I'm not sure if it's legal to log into each other's steam accounts).
Otherwise a dev/admin could help here. But then your friend should come here in person.^^