Report the sub-letting to the local authority (if it's rented out via them or a Housing Association?). If so, he'll not only get done for benefit fraud and the authority/HA can pursue a civil prosecution, but the authority can also (and will probably be successful in) pursue a criminal prosecution.
If it's as straightforward as the above, it'll be a pretty easy case to evidence for...
He'll have to pay back all that they've gained through the benefit fraud, rent money through sub-letting, let alone the legal costs...