Praying in a home that was built using an interest-based loan
Taking out riba-based loans is emphatically haraam, because of the warnings and curses that have been narrated concerning that, whether it is for building a house, buying a car or anything else.
The one who has fallen into this sin must repent to Allaah and resolve never to do anything like that again.
But he may use and benefit from what he bought or built with this loan, such as a house etc, because the money that was borrowed on the basis of riba belongs to him, and what he does with it of buying or building is valid.
Based on that, there is nothing wrong with you entering his house and praying there, or eating his food, so long as you call on him to repent and regret what he did before.
See also the answer to question no. 22905
And Allaah knows best.