You are allowed to accept the payment regardless of whether he has borrowed money on interest or not. The Prophet's companions - including the pious caliph Umar - used to receive payments from Christians and Jews who used to sell alcohol and other items forbidden in Islam. So, you need not be concerned with the source of her money. If however on this ground you want to forgive and write off the loan, that is another issue. That indeed would be most desirable.
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