He works in a riba-based bank and he wants to get out but he cannot find any alternative

Praise be to Allah.

Firstly 

Working in a riba-based bank is undoubtedly haraam, because riba is haraam and it is haraam to help with it in any way whatsoever. That has been explained in the answer to question no. 26771. 

Based on this, you have to repent to Allaah, may He be exalted, and hasten to leave this work as soon as you can. 

We do not say that you have to stay in your job until you find an alternative, rather we say that you should hasten to leave it whilst looking for an alternative, and have faith that Allaah will compensate you with something better, for if a person gives up something for the sake of Allaah, Allaah will compensate him with something better than it, as the Prophet (peace and blessings of Allaah be upon him) said: “You will never give up something for the sake of Allaah, may He be glorified and exalted, but Allaah will replace it with something that is better for you than it.” Narrated by Ahmad (21996). Al-Albaani said: Its isnaad is saheeh according to the conditions of Muslim. 

So look hard and be serious about it, and ask Allaah to help you. Be certain that Allaah, may He be exalted, is too generous to cause you to be lost. Allaah says (interpretation of the meaning): 

“And whosoever fears Allaah and keeps his duty to Him, He will make a way for him to get out (from every difficulty).

3. And He will provide him from (sources) he never could imagine. And whosoever puts his trust in Allaah, then He will suffice him. Verily, Allaah will accomplish his purpose. Indeed Allaah has set a measure for all things”

[al-Talaaq 65:2-3]

In Musnad al-Imam Ahmad (205) it is narrated that ‘Umar ibn al-Khattaab (may Allaah be pleased with him) said that he heard the Prophet of Allaah (peace and blessings of Allaah be upon him) say: “If you put your trust in Allaah in the true sense, He will grant your provision as He grants to the birds, who go out in the morning hungry and come back full.” Classed as saheeh by al-Albaani in al-Saheehah (310). 

Secondly: 

If a person meets his Lord with righteous deeds such as prayer and fasting, but he also meets Him with the sin of riba, if he had repented from it, then his bad deeds will be turned into good, as Allaah promised in the verse (interpretation of the meaning): 

“Except those who repent and believe (in Islamic Monotheism), and do righteous deeds; for those, Allaah will change their sins into good deeds, and Allaah is Oft‑Forgiving, Most Merciful”

[al-Furqaan 25:70]

But if he meets Him persisting in riba without having repented, then this is a person whose righteous deeds are mixed with bad deeds, and it is up to Allaah: if He wills He will punish him and if He wills He will forgive him, as He says (interpretation of the meaning):  

“And (there are) others who have acknowledged their sins, they have mixed a deed that was righteous with another that was evil. Perhaps Allaah will turn unto them in forgiveness. Surely, Allaah is Oft-Forgiving, Most Merciful”

[al-Tawbah 9:102] 

“And others are made to await for Allaah’s Decree, whether He will punish them or will forgive them. And Allaah is All-Knowing, All-Wise”

[al-Tawbah 9:106]

We ask Allaah to help and guide you. 

And Allaah knows best.