He has repented from dealing with a riba-based bank but he does not know what his capital was

Praise be to Allah.

Your friend should contact the bank to find out how much the interest was that was added to his capital, then he should get rid of this interest by spending it on charitable causes. See question no. 292 and 2370. 

If he does not know how much it is exactly, he should pay what he thinks is most likely to be the amount of interest that he took, and Allaah does not burden any soul beyond its scope. 

With regard to the interest-based loans that he took and has paid off, that belongs to him. It is sufficient for him to repent and regret what he has done, and to resolve not to repeat such haraam actions. 

We praise Allaah for helping him and guiding him away from these haraam interactions, and we ask Him to bless him in his wealth and to provide for him of His bounty. 

And Allaah knows best.