Apply and take out the latest Encumbrance Certificate from the jurisdictional sub-registrar office where the property is situated, if it reveals that the property for which you had given a POA in favour of your friend has been sold, the date on which it was sold for what consideration and in whose favour will be stated, if it is not then the POA has not been misutilized by your friend.
None the less before initiating any further steps as above, you can proceed to revoke the POA by executing a Revocation Deed and getting the document duly registered at the sub-registrar's office, after revocation is duly registered, take out a PUBLIC NOTICE in two leading newspapers advertising to the common public at large, that the POA Dt; so and so, has been revoked or cancelled by you and henceforth any person/s entering into any kind of agreement or conveyance deed or in particular any document affecting the schedule property in any manner whatsoever, they will be doing so at their own risk and you will not be liable in any manner whatsoever and the same is not binding on you either.
this process would be sufficient