Does helping kafirs against muslims under life threat nullify one's islam? Can't he repent? Can't he do the work & say sorry to muslims later & repent?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

It is not permissible for a Muslim to support the non-Muslims against the Muslims no matter how big or small this support is, so whoever does this out of his own free will, he is immensely sinful but he does not go out of the fold of Islam just because of this support. Also, if he did this for any worldly benefit while his heart is content with belief, then he does not go out of the fold of Islam as well.

Al-Qurtubi said in his Tafseer (interpretation of the Quran) while commenting on the story of Haatib ibn Balta’ah and the verses that were revealed about him at the beginning of Chapter Al-Mumtahinah [Quran 60]; which is the saying of Allaah (which means): {O you who have believed, do not take My enemies and your enemies as allies, extending to them affection.} [Quran 60:1]: “The saying of Allaah {…extending to them affection} meaning outwardly because the heart of Haatib was sound; the evidence is that the Prophet said to the Companions about him: ''As regards your companion (Haatib), he is truthful.” This is a (religious) text stating that his heart was sound and that his belief was intact … So, whoever frequently follows the faults of the Muslims and informs their enemy about their faults and their news, he does not go out of the fold of Islam if he did so for a worldly benefit while his belief was sound, in the same manner as did Haatib when he intended to get support (from the non-Muslims) but did not intend to apostate from Islam.” [End of quote]

What you mentioned that that person will apologize to the Muslims after that, is not a sound reason for him for doing so, as it is a sin that is not committed against the Muslims only, but also against Allaah. How can a Muslim know that he will live until he repents from that sin?

Nevertheless, if a Muslim does so and then he sincerely repents to Allaah, then Allaah will accept his repentance; for more benefit, please refer to Fatwa 85286.

In case that person was threatened to be killed if he does not do so, and he predominantly thinks that they (his enemies) would really kill him if he does not do so, then he is compelled, and compulsion removes the competence of entitlement to carry out the Islamic obligations as Allaah Says (what means): {Whoever disbelieves in [i.e. denies] Allaah after his belief except for one who is forced [to renounce his religion] while his heart is secure in faith. But those who [willingly] open their breasts to disbelief, upon them is wrath from Allaah, and for them is a great punishment.} [Quran 16:106]

In such a case, he is obliged to help them only to the extent of repelling the threat, and in case he can repel this threat by Tawriyah (i.e. saying something which has more than one meaning and intending a meaning different from what the listener is likely to understand), then he is not permitted to speak explicitly.

For more benefit, please refer to Fatwa 85727.

Allaah Knows best.