There is so much ignorance regarding benefits, housing benefits etc already on this thread.
If your relative is subletting, regardless of where he lives, he is committing fraud.
DLA & PIP eligibility does not enable access to housing benefits because they are not means tested. You can work and be in receipt of these. However, if the claimant is in receipt of DLA/PIP there are additional premiums payable, such as increased housing benefit or council tax reduction when a child in in receipt of DLA or the disabled adult has a sleep-in carer. The rules are complex. If he is on UC or income based ESA he is entitled to claim housing benefits IF he fits the rigorous eligibility criteria.
Don't confuse your relatives alleged housing benefit fraud with any disability benefits he may be receiving. Unless he claims means tested benefits he cannot claim housing benefits.
To claim PIP, ESA or UCs new style ESA the claimant has to provide recent relevant evidence of their illness or disability (hospital letters, physio, MH evidence etc). The application process is thorough and rigorous, and extremely stressful as it isn't always easy to get this evidence.
If a tip off to potential benefit fraud is received the benefits are stopped until it has been thoroughly investigated. This can take many months. A false claim of fraud can be devastating and result in loss of accommodation, increasing new debts, inability to afford essentials.
If you want to report for housing benefit fraud you need to contact the local authority that is paying the housing benefits.
If you want to report benefit fraud you need to contact the DWP.
Local authorities and the DWP work together to tackle fraud and they will share evidence if there is potential fraud.
Think carefully before you report suspected fraud and think about why you want to report. As a fraud investigator (not benefits related) until I lost my sight I have led many 'tip off investigations' and the devastation caused by malicious tips off is truly awful.