When booking award flights for others I take care to make them aware of how points work and make sure they understand that award flights are not free flights.

In this case I would ask the friend to pay back the value of the points or try to cancel the change. OP should be able to call Delta and inform them that the original ticket was authorized, but not the change.

In this situation I would expect Delta to reimburse the miles and fee as the activity was unauthorized.