Unfortunately, anyone who is, for one reason or another, afflicted with depression experiences such a powerfully painful reality that this is the last thing he wants to hear. The lethargy born of depression blocks all normal impulse for escape from this uncomfortable state, all incentive to get up and do something to help oneself. The idea that the person is being won over by the yetzer hara only served to intensify the feelings of self hatred and disgust and despair.
In a sense, depression is anger turned in against the self, an eyn hara turned inwards. A curse of despair.
What can we do to release the depressed person from his prison?
Ultimately the depressed individual has to move himself. He has to desire an emotionally healthy state more than the depression, so much that he will hear his own sechel and take steps to help himself. To do that he has to love himself and believe in himself- to recognise his own potential greatness.
To help him do this, we must not condemn with judgements such as "mentally weak", "emotionally unstable", "selfish", "nebach", or any such derogatory labels. When the depressed person has become free of his pain and reached a higher level of maturity, he will be able to look back and make any painful assessments for himself. While he is depressed, however, we should focus on his positive attributes. We need to build him and awaken his sense of self worth, without pampering and feeding any desire for sympathy or attention or approval from outside which might exist.
It is very important to evaluate the problem in the kindest, most positive light. The depressed person is suffering greatly. He may even feel suicidal. To revive his soul could constitute the mitzvah of pikuach nefesh. You can tell her that she possesses a special kind of soul. Her soul is sensitive, creative, aware to a very high degree of the environment, in an aesthetic sense and otherwise. This is almost always true for the kind of individuals who suffer regularly from depression. Tell her that the sitra achra is davka trying to block her creativity with this inner directed pain, davka trying to fool her into believing that she is worthless when the exact opposite is true.
Tell her also that she possesses the ability to win over the pain, to fool it, to give it the slip. Hashem has given her the inner power to transcend and escape the mud.
If you had in your hand a fifty carat diamond, you would not let it slip carelessly into the mud. You would certainly not put in there intentionally. If you knew it was hidden in the mud at your feet, wouldn't you scrabble on your knees, refuse to budge from that spot until you had recovered that stone? When you do, wouldn't you raise it aloft, filled with delight, wipe off the grime, and admire your find with joy and relief.. and as soon as possible, wash the gem and rub it to a high shine? If we could only believe that our neshamas were as precious - no, really far more precious than that diamond, we would feel the necessary motivation to rescue ourselves from depression.