Answer
(Fatwa: 592/592=L)
In income the majority is counted. If the majority of his income is haram (unlawful) then it will be considered haram and if the majority of his income is halal (lawful) then it will be considered halal. So in the above-mentioned case, when you are sure that the major part of his income is unlawful it will not be allowed for you to attend his walima feast:
اٰکل الربوا و کاسب الحرام أھدی الیہ و أضافہ و غالب مالہ حرام یا یقبل و لا یاکل ما لم یخبرہ أن ذلک المال أصلہ حلال ورثہ أو استقرضہ۔ و ان کان غالب مالہ حلالا لا باس بقبول ھدیتہ و الأکل منھا (عالمگیری: 5/343)
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband