honestly, big but shit town. The promenade along the Rhine is cool to walk for a couple miles especially in the district with all the embassies and stuff. Otherwise if you're going these days you'll have all the typical German winter markets with 1 meter long sausages and stuff.
could be worse, he could be going to Leverkusen or Bochum
Have a look at Cologne while you're there, the district around the cathedral and along the Rhine is quite small and there are good spots for pictures. Statues and stuff.