Termination Due to Non-Payment of Loan
First of all, please tell us what you want from us: a draft of a letter terminating an employee or advice on whether you can terminate an employee for non-payment of a loan to the company? If the first is your choice, then you may write to the employee stating that since they have not paid back the loan, they are hereby terminated from service. Anyway, they are going to face the consequences of termination, and that they should face.
If you are okay with the second one, we have the following advice based on certain questions.
Questions to Consider Before Termination
Have you ever communicated this matter to him? What was the arrangement when he was given a loan? Was there any arrangement that the amount would be recovered from salary in installments? If yes, why didn't you recover it from his salary? If there were no such arrangements but the loan was given with an understanding that the employee would pay it back once he gets some money from other sources, have you requested him to pay it back at least once in six months after it has become overdue? If the answer to the above is No, then it is your fault that you did not take the initiative to recover the amount advanced.
Options for Recovery
Now, the only option available is to give him a letter stating that an amount of Rs..... is overdue and the same should be paid back in..... months, failing which legal action as deemed fit shall be taken against him.
Here, the time that you should give should equate with the amount involved and the salary and other income of the employee. Obviously, you should give more time to an employee whose monthly take-home salary is less and where the amount involved is huge. You can also think of salary deduction. Terminating without giving him an opportunity to be heard is unfair.