I know a benefit fraudster?

My friend is claiming over �2,000 per month on benefits (ESA and disability under two different identities) and is working (8 hours per day) for a homeless charity, whilst also donating around �1,800 to various homeless charities each month. I'm wondering of how I can prevent this, the reason due to this becoming a sincere detriment to other individuals attempting to apply for benefits (especially those who are in dire need of them).