If one’s dealings with riba-based banks is limited to depositing money in current accounts without any interest because there are no Islamic banks and because companies need to do that in order to protect the money and make it available for trade, there is nothing wrong with it. 

But if the dealings with them involve haraam things such as borrowing from them directly or in other ways such as buying through them or opening a line of credit with them etc, then these dealings are haraam and it is not permissible for anyone to engage in them, because that is cooperating in sin and transgression, and bearing witness to riba and approving of it.  

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”

[al-Maa’idah 5:2]

And Muslim (1598) narrated that Jaabir (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) cursed the one who consumes riba, the one who pays it, the one who writes it down and the two who witness it, and he said: They are all the same. 

Based on this, if the company deals with the banks in this manner, then you did well to leave this job, and we ask Allaah to accept your repentance. But you were wrong to teach it to someone else, because that is teaching and helping him to do something haraam. In that case you should explain the shar’i ruling to this brother and advise him, as well as repenting to Allaah. If he responds then praise be to Allaah, and if he does not, then you have still done what is required of you. 

We ask Allaah to guide you. 

And Allaah knows best.