Over two week unwashed coffee cup

A friend just contacted me on Skype.  Until this morning he was here visiting, but then he flew back to his country and now he wanted to say thank you for looking after him and stuff.  He said that he had a coffee cup in his kitchen sink he hadn't washed when he left more than two weeks ago.  Now it has grown fungus or something and his whole apartment stinks he says.  Gee, I couldn't stand the idea of leaving it even one morning unwashed, imagine two weeks!