how do you... or do you??

How do you tell your SO that you know they're depressed? Or do you even tell them?? My husband told me his friend called him saying he was struggling bad with depression. I asked him if he felt that way and he said sometimes. I know he is but he changed the subject quickly. We move around a lot and he doesn't stay at jobs very long. He can't seem to be happy no matter where he is. What would you do?