This a good question, and the spiritual-scientific answer to it is:
Even if a person commits suicide just once, the repercussions of this act are so grave, that one will have to do the same for several lives to come.

The karma (seed) of committing suicide is so serious, as a result (fruit) of which, one has to keep committing suicide, life after life, for atleast seven lives.
So, when a person commits suicide in their current life, there is a possibility that the suicide of this life may be the karmic effect of the suicide committed in the past. In such a case, the event is inevitable. When it’s time for the fruition of the past karma, even if the person doesn’t wish to, the circumstances so arise that one is drawn into committing suicide.

If killed once, why does one have to kill themselves several times?
Let’s understand this through an example.
When you throw a ball from a height of about three feet, it will bounce back up and then drop, will again bounce up and then drop, isn’t it? So, although the ball was thrown just once, depending on the intensity with which it was thrown, the ball is forced to bounce up and down multiple times, before it ceases to bounce. This is a natural process, a natural principle of motion.

Similarly, if a person commits suicide just once, then this act is perpetuated for atleast seven more lives.
That is why, all Enlightened beings say, ‘endure whatever life brings forth before you, regardless of how severe the suffering may be’, because by enduring the suffering, the karma comes to an end; but if we get afraid and take a drastic step as killing one’s self, severe karma gets created, for which we’ll have to undergo severe suffering, many times more than the initial suffering that we had got scared with.

Param Pujya Dadashri explains, "A person may die by committing suicide; but he will have to come back to pay off his debts and obligations.” So, there is no escape, as such!
Can we help those who have taken their life?
Yes. Even though they are no longer with us, we can pray to their Soul, to help them come out from the cycle of suicide. Rather than worrying, it is more beneficial to pray to their Soul. They will experience peace wherever they may be.

Pray that they come across an Atma-Gnani (one with full knowledge of the Self). Only these elevated beings will be able to set them free from their entrapment and suffering of karma.
Pray that they acquire the right understanding and be free from suicidal thoughts in the future.
These are useful solutions. By implementing them, you will feel at peace, and so will the other person as your prayers reach him.

To Conclude: The Soul of those who have taken their life experience great suffering. The best way to help them is through prayers. To liberate ourselves from all sufferings and miseries, one must meet Atma-Gnani!

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Author's Bio: 

Ambalal M. Patel was a civil contractor by profession. In June 1958, spontaneous Self-Realization occurred within Ambalal M. Patel. From this point on, Ambalal became a Gnani Purush, and the Lord that manifest within him became known as Dada Bhagwan. A Gnani Purush is One who has realized the Self and is able help others do the same. Param Pujya Dada Bhagwan used to go from town to town and country-to-country to give satsang (spiritual discourse) and impart the knowledge of the Self, as well as knowledge of harmonious worldly interactions to everyone who came to meet him. This spiritual science, known as Akram Vignan, is the step-less path to Self-realization.