All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
It is not permissible for you to join in partnership with your friend because he obtained his share from a loan with interest. In joining with him in partnership, you are confirming his wrong doing and helping him in doing so. Allaah says (which means): {Help you one another in virtue, righteousness and piety but do not help one another in sin and transgression.}[Qur'an 5:2]. Moreover, the Prophet said: "He who sees something abominable should change it with his hand (physically); and if he is unable to do so, then with his tongue (verbally), and if he is unable to do so, then he should (abhor it) from his heart, and this is the weakest level of faith." [Muslim] In order to forbid evil [in your case] you have to refrain from dealing with him. Besides, you have to advise your colleague; it might be that Allaah would guide him thanks to you. The Prophet said: "I swear by Allaah! If Allaah guides one single man (to Islam) by your efforts, that will be better for you than possessing an abundant number red camels (a very expensive breed of camels)." [Al-Bukhari and others]
Allaah knows best.