If he is 'officially' at the address [even if he is not living there] it is difficult to determine.
Once his tenancy agreement ceases, he hands back the keys, removes *all* of his stuff........then he'll be committing an offence. At the moment, he could claim that he is staying somewhere temporarily and will be returning to the property.
Perhaps looking for details on how HB is awarded and if there is a 'cut-off' point for how long you are allowed to be absent from a property before HB is cancelled might be an idea. But if your friend wants this person out, it may be best to seek proper legal advice.