It is not permissible to help with riba in any way, whether that is writing it down, witnessing it, working as a security guard, programming, looking after equipment or any other kind of help, because Allaah says (interpretation of the meaning):
“Help you one another in Al-Birr and At-Taqwa (virtue, righteousness and piety); but do not help one another in sin and transgression. And fear Allaah. Verily, Allaah is Severe in punishment”
If you can avoid that, then your work is permissible, otherwise you should look for another job that is permissible. Be certain that provision comes from Allaah, and that it is acquired by obeying Him, as He 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).
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”
It is known that many companies deal with riba, and that whoever specializes in a field such as yours will encounter some difficulty in looking for permissible work. But that cannot compare with the great sin that is borne by the one who helps others to commit a sin for which Allaah threatens those who do it with war.
So fear Allaah and beware of His punishment. Make sure your food is halaal, for every body that is nourished from haraam sources, the Fire is more fitting for it. (Hadeeth narrated by al-Tabaraani and by Abu Na’eem in al-Hilyah; classed as saheeh by al-Albaani in Saheeh al-Jaami’, no. 4519.
And Allaah knows best.