Is a son sinful if his mother registers an apartment for him that she bought with money deposited in the bank for a long time?

If the price of the apartment is mixed with lawful (the original capital) and unlawful (riba-based interest), then it is not forbidden for the questioner to accept it, because the unlawful debt was incurred by the mother, not inherent in the apartment itself. The questioner must advise her mother to repent from dealing with usury and to dispose of the usurious interest by spending it on charitable causes.