Praise be to Allah the Lord of the Worlds.
We recommend that you keep advising your friend and reminding him that he is required to repent and give the money to who it belongs as soon as possible since death is inevitable and one doesn`t know when his time comes. What a horrible end will it be for someone to die having eaten up people`s wealth?!
It is imperative that he calls those from whom he had stolen that money and ask them to give him time to pay it back, even by installments, since they may forgive him and that may enable him to make sincere repentance to Almighty Allah.
As for the food you had eaten and he paid for, it isn`t lawful so long as you are sure that he paid for it from that ill-gotten money. You should have turned him down so that he realizes the gravity of his sin and that you avoid eating food from unlawful money. However, since you have eaten that food, you must make repentance and ask Allah to forgive you.
And Allah The Almighty Knows Best.