I know someone who will commit suicide. What should I do?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

We clearly state that committing suicide is strongly forbidden whatever the motive/reason is. Allah Says (interpretation of meaning): {… And do not kill yourselves (nor kill one another). Surely, Allâh is Most Merciful to you.}[4:29].
Also the Prophet (Sallallahu Alaihi wa Sallam) said: "Whoever purposely throws himself from a mountain and kill, himself, will be in the Hell-Fire falling down into it and abiding therein perpetually forever; and whoever drinks poison and kills himself with it, he will be carrying his poison in his hand and drinking it in the Hell-Fire wherein he will abide eternally forever, and who ever kills himself with an iron weapon, will be carrying that weapon in his hand and stabbing his abdomen with it in the Hell-Fire wherein he will abide eternally forever" . [Reported by Imam al-Bukhari ].
The above-stated Hadith indicates that killing oneself is one of the major sins that cause one to reside in the Hell-Fire forever. And one who kills himself will be tortured in the same way and means by which he killed himself, in addition to being an inhabitant of the Hell-Fire.
Allah, The Great and Almighty, has banned any form of despair or desperation and stated that despair is one of the disbelievers' characteristics. He Says (interpretation of meaning): {"O my sons! Go you and enquire about Yûsuf (Joseph) and his brother, and never give up hope of Allâh's Mercy. Certainly no one despairs of Allâh's Mercy, except the people who disbelieve."} [12:87].
He also Says: {Ibrâhim (Abraham)] said: "And who despairs of the Mercy of his Lord except those who are astray?"} [15:56].
As a result, know that committing suicide constitutes a bad action and leads to a bad end, so you have to avoid it no matter what the problems are.
One has to have a great hope in Allah's Mercy which encompasses everything; one has to have stronger relation with his Lord through good deeds, supplication and prayers so that He helps one love faith, hate disbelief and make one among the righteous. One has to seek Allah's help to help him avoid what is bad and do what is good.
One has to know that life is a good Divine blessing that we are to use it in doing good deeds in order to live a happy life.
Allah Says (interpretation of meaning): {Whoever works righteousness, whether male or female, while he (or she) is a true believer (of Islâmic Monotheism) verily, to him We will give a good life (in this world with respect, contentment and lawful provision), and We shall pay them certainly a reward in proportion to the best of what they used to do (i.e. Paradise in the Hereafter).} [16:97].
In this context, the Prophet (Sallallahu Alaihi wa Sallam) said: "The best human beings are those who live long and do good deeds; and the worst people are those who live long and do bad deeds" . [ Ahmad and al-Hakim ; al-Tirmizi said it is a sound Hadith].
Then, why resort to suicide which brings down on you Allah's hate and punishment?!!
Know that a sensible person does not think of suicide, since such an action will lead to less Here and Hereafter.
At the end, we advise you to accompany the righteous, those who urge you to do what is good and to avoid what is bad. We also advise you to attend gatherings of knowledge and al-Zikr, to observe prayers in their fixed times, to have patience, and then, to leave your affairs to Allah's providence.
He Says: {…, but give glad tidings to As-Sâbirin (the patient ones, etc.). Who, when afflicted with calamity, say: "Truly! To Allâh we belong and truly, to Him we shall return". They are those on whom are the Salawât (i.e. blessings, etc.) (i.e. who are blessed and will be forgiven) from their Lord, and (they are those who) receive His Mercy, and it is they who are the guided-ones.} [2:155-157].
We ask Allah to grant all of us more success and pray to Him to help you avoid each and every harm.
Allah knows best.