Hazrat once said that if a person takes an interest-based loan from a a bank and then conducts a Halal business with it, then even though he would be incurring the sin of paying interest, his income would be halal and it would be permissible to eat at his home. However, if a person loans money to others and charges interest on it, and majority of his income comes from that source, then his income is Hara'am and one should avoid eating at such a person's home.