Brother it is haram to commit suicide in Islam, if a person commits suicide they are then to face punishment in hell for the rest of life after death as stated in the Qu'ran:
“....and do not kill ourselves; surely Allah is Merciful to you. And whoever does this aggressively and unjustly, We will soon cast him into Fire; and this is easy for Allah.”
(Surah an-Nisā’ 4:29-30)
Listen if a kafir (Non believer) doubts about the message being sent from Allah onto mankind and doubts on life after death, you should not tell them to kill theirself or commit suicide in anyway.
Clearly the message was sent upon mankind right
So when the non believer doubts in what Allah has sent
When there death actually approaches them
And the first stage will be when in the grave, this is when the two
Angels come face to face with the soul and ask questions:
1. What he used to believe
2. What's his religion
3. Who's his lord
4. Who's his prophet
The believer will give correct answers whereas the non believers will struggle to answer, due to not having faith they will face severe and painful punishments.
Overall I am trying to say is don't tell them to commit suicide, the soul will find out the truth once leaving this world. Our duty is to provide the non believers with alot amount of information as possible, even if they refuse, when they pass on, Allah will say to them did this certain person not come to you and pass you the message, the soul will say yes but did not believen it, the non believer soul will ask Allah to recarnate him/her just to put things right but Allah will refuse to do so. I can't find the exact surah when it mentioned similarly to what I just stated but do you research and we will be put onto the right path Insh'Allah.
Any questions, inbox me or just reply to me.