Eating food from, or buying from, a Muslim business that also sells alcohol

Answer

If the entirety of a person's wealth is from a haram source, it is not permissible to eat from what they serve using that wealth; if there is a possibility the food served comes from an alternative halal source of income, it is permissible. If it is a food stall and the food is bought to eat elsewhere, the food is halal (provided all conditions of halal food are met) and the purchase is valid. If it is a restaurant where one would eat on the premises while alcohol is served and consumed around them, it is not permissible to remain in such an environment without trying to prevent the sin from occurring, although the food and the purchase themselves remain valid. Either way, it is better not to buy from such a business at all, since doing so supports the continuation of the practice -- if possible, advise the owner privately and respectfully to stop, and if they refuse, let them know you will no longer buy from them, in the hope of encouraging change.