Reference: Fadhaa.il al-A’maal – Page 7
Question: What is the ruling about working with someone whose sustenance is from prohibited wealth? And what is the ruling of eating [from the provision] of this person at gatherings?
Response: If it is certain that all of this person’s income/wealth is haraam, then it is not permissible to eat from his provision. If, however, it is a mixture of [both] haraam and halaal – then there is no harm in eating [from his provision]. That is because you do not know whether he fed you from this or that. And the base principle [here] is safety [from that which is prohibited, unless something contrary is conclusively established] in shaa.-Allaah.
And [don’t forget that] the Prophet (sal-Allaahu ‘alayhi wa sallam) himself would eat from the provision of the Jews.
And with Allaah lies all success.