Reserve deputy sheriff working private security for pay

A friend of mine is a reserve deputy sheriff (means he earns no pay but volunteers his services when needed by local law enforcement and has authority to write tickets, arrest, direct traffic, etc.).  Through this organization, he is also able to wear the uniform and be paid to work private security at various events (local festivals/fairs, weddings, etc.).  This pay comes directly to him from the sponsors of the event.  It does not pass through the sheriff's reserve or sheriff's department and is often paid in cash.  I know the income earned in this capacity is subject to Federal and state income taxes, but I am wondering if it is subject to self-employment tax as well.  I cannot seem to find a definitive answer on this in my browsing of these message boards and IRS publications.  Any help is appreciated!  Thank you!