Is the second borrower a participant with the first borrower in the sin, or is one of them more sinful than the other, knowing that the second borrower borrowed to meet his brother's need?

Borrowing with interest is usury and a grave sin. A Muslim is not permitted to engage in it except in a state of extreme necessity, similar to consuming dead meat or pork for one who is compelled. As for making excuses when there is no necessity, that is among the deceptions of Satan. One who has borrowed with usury must repent to Allah. However, dealing with a person who has borrowed [with interest] (through a gift or a loan) is not problematic, unless it involves assisting in sin.