Hi
1) You need not worry at all as you have effected a simple mortgage by registering the document at sub-registrar office.
2) Your friend cannot unilaterally cancel the mortgage at the sub-registrar office as Unilateral cancellation of mortgage deed is not permitted in law.
3) You will need to be physically present and sign the mortgage cancellation deed along with the borrower for cancellation of mortgage deed.
4) Also i presume you have the original title deed with you.
5) If you anticipate that your friend is planning to sell the property without repaying the loan / or is adopting dubious ways, the best way is to recall the loan and ask your friend to repay the monies failing which you take over the property by filing a Summary suit under Order 37 rule 1, 2 CPC.
6) A summary suit will be decided in a maximum of 3 months in any court in India.
Hope this helps