Reference: Fataawa wa Rasaa.il Mukhtaarah – Page 503
Question: What is the ruling regarding working with someone whose provision is from haraam earnings? And what is the ruling regarding eating from the provision of this person on occasions of celebration?
Response: If it is confirmed that the earnings of this person is all haraam, then it is not permissible to eat from his provisions. If, however, his earnings are a mixture of halaal and haraam – then there is no harm in eating from his provisions. That is because you do not know whether he has fed you from this or that – and the basic principle is one of being safe (precautious), in shaa.-Allaah. And [it is known] the Prophet (sal-Allaahu ‘alayhi wa sallam) would eat from the food/provisions of the Jews.
And with Allaah lies all success.