Symptoms can include tiredness, lethargy, a general melancholy not necessarily associated with any particular event or events, increasing or sudden self-exile from social/family groups, increased dependence on legal and illegal substances - caffeine, alcohol, "narcotics" -, insomnia, excessive sleeping, confusion, decreased motor abilities, physical self-abuse - cutting, burning etc.-, suicidal thoughts, increased/decreased self-hygine, and on and on.
Some, all or indeed none of these symptoms may be present in a depressive, and this list is in no way complete.
It is important to remember that a real depressive cannot just "pull their socks up", and may not be able to attribute their feelings to an event, as depression is caused by various chemical imballances in the brain. The sufferer may not even be aware that they have a problem at all (which I was not when I was diagnosed - I went in to find out why I was tired and feeling unmotivated all the time and I came out with a script for Prozac. I also then went on to exhibit most of the symptoms I've detailed above over the course of the next 2 years or so, and even now struggle with the "lower end" of the symptoms).
My best wishes go to whomever you are asking on behalf of, and my best advice for the depressive or the supporter is to never give up, no matter how low, or unworthy the feelings.