Someone I know is suffering from extreme pain and a killer is coming to kill him. Is suicide permissible painlessly.
I'm so sorry to hear about the extreme suffering of your loved one 💔 and I pray that Allah makes it easy for him while taking away his pain and anguish. Ameen.
In no instance whatsoever is suicide allowed in Islam
In fact in any trial we go through as a Muslim, however painful, we should seek refuge with Allah and bear patience. There's a severe warning in Islam that whosoever commits suicide they will be tormented for eternity in hell with the means that they killed themselves with. We have to bear in mind as Muslims that however terrible the suffering is in the life of this world; the suffering of the hellfire is a lot, lot worse.
It was narrated from Abu Hurayrah (may Allaah be pleased with him) that the Prophet (blessings and peace of Allaah be upon him) said: “Whoever throws himself down from a mountain and kills himself will be throwing himself down in the Fire of Hell for ever and ever. Whoever drinks poison and kills himself will be sipping it in the Fire of Hell for ever and ever. Whoever kills himself with a piece of iron will have that iron in his hand, thrusting it into his belly in the Fire of Hell for ever and ever.” Narrated by al-Bukhaari (5442) and Muslim (109),
It was narrated from Thaabit ibn Dahhaak (may Allaah be pleased with him) that the Messenger of Allaah (blessings and peace of Allaah be upon him) said: “Whoever kills himself with something will be punished with it on the Day of Resurrection.” Narrated by al-Bukhaari (5700) and Muslim (110).
It was narrated that Jundub ibn ‘Abd-Allaah (may Allaah be pleased with him) said: The Messenger of Allaah (blessings and peace of Allaah be upon him) said: “A man among those who came before you was wounded. He panicked and took a knife and cut his hand, and the bleeding did not stop until he died. Allaah, may He be glorified and exalted, said: ‘My slave hastened his death; I have forbidden Paradise to him.” Narrated by al-Bukhaari (3276) and Muslim (113).
May Allah grant our brother ease and take away his suffering. Ameen.
If there's a threat to his life as you mentioned that someone is coming to kill him; I would also advise as a brother already has, that he phones the police as it's something very serious 💔