All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
It is impermissible for you to lend your friend this money in return for taking a share of his earnings or any other form of payment that exceeds the original amount of the loan; this is plain Riba (interest). Please refer to fatwa 3522.
Allah knows best.