It’s never easy to speak about mental health issues so remember, it doesn’t always have to be a long conversation. There are other ways of showing support. One can send an email message. One can send a message on face book but not on the public page, of course. One can simply call and ask; ‘How are you doing?’ Small gestures like this can make a big difference.
If you think that someone is behaving differently from their usual manner of behavior, that person could be unwell so how about making it clear that you have noticed that he doesn’t seem his usual self at that time and wonder whether there is anything you can do to help. Make it clear that if your friend ever wants to talk you will be there for him or her. Or, offer practical help. Try not to avoid the subject. People with a mental health problem often mention the fact that friends and colleagues avoid them simply because they have no idea of what to say. You can ask how things have been and how they are doing, you know. They might even welcome the attention. If that person is returning to work or to your social group after a period of illness, let him know how glad you are to see him return and maybe even suggest a further chat at a later stage as it is so important to the person concerned.
People want someone to listen to and not necessarily to give advice. If you jump to conclusions and give advice, it is equivalent to not listening to that person. If you are sure that the person needs urgent help, turn to a professional for advice.
It is important to empathize but saying that you felt equally sad when your dog died, will most definitely sound like the most trivial comparison in the world. You can of course say that you have no idea what it must be like for that person as your experience cannot possibly be the same as theirs.
On no account tell that person to pull himself/herself together or that it will seem better in the morning. A person in trouble is good at picking up body language so try not to keep glancing at your wristwatch or smartphone too often to check the time.