What is the ruling on visiting a friend who lives in a house bought with a usurious loan, and dealing with him, or staying in his house? And does the ruling differ if my friend condones dealing in usury?

There is no objection to visiting the owners of houses purchased with money borrowed through usury (riba), and it is permissible to stay in them and benefit from them, because the sin of the usurious loan is attached to the borrower's liability, not to the money itself. If these people deal with usury, you should forbid them from it and explain its ruling to them. If they persist in dealing with usury and shunning them would deter them from this evil, then shun them.