IMHO it is important to make sure the person realizes that they are looking through a distorted window at life brought on by a biological condition. Have them try to identify what the most prevelent thoughts are that they have while feeling this way and then relate your own feeling about those thoughts as a reference point, that is as long as these thoughts don't depress you as well.

Share your own feelings on things you might not normally talk about to try and redirect their thinking.