What should be done with food entrusted to someone for a person who has been absent for a long time, and it is unknown when he will return, and the food will spoil? And is it sinful if someone eats it or gives it to someone else to eat?

If the owner of the food is absent and you fear it will spoil, you may sell it and keep its price for its owner, or you may eat it or feed it to whomever you wish, and guarantee its price to him. This is because found food that is feared to spoil is, in the hands of the finder, like a deposit. So, the finder has the choice between selling it or eating it while guaranteeing its price, and should not leave it until it spoils. If he leaves it until it spoils, then it is his responsibility due to his negligence.