Muslim must give back the rights of others in any possible way. Debts should be written to save the rights of others as we read in Quran (O you who believe, when you take a loan for an agreed time, you should write it down) (2:282).
The person who gave the money should remind the person who took the loan and present evidence including witness or any other supporting evidence. If the person who took the money denies remembering the loan, he should be asked to take an oath by Allah that he did not take any amount.
Claimant needs to prove while opposite party needs to take oath. البيّنة على من إدّعى واليمين على من أنكر.