Is it permissible to eat the food of a person who works in a pig-rearing stable and to interact with him, knowing that his wages are his only source of income?

It is not permissible for a Muslim to work in pig farming, and what he earns from it is religiously forbidden. Your friend must leave this work and seek another job, because whoever fears Allah, He will provide for him from where he does not expect. As for eating from ill-gotten wealth, it is not permissible if all the wealth is unlawful or if it comes directly from unlawful money. However, if it is mixed, it is permissible with karaha (dislike) according to the majority of scholars.