My friend is interested in joining the reserves or guard to help with college and get a little adventure but he's worried about being deployed, can an only child be deployed if he or she is currently in the reserves or guard?

yes he can
being an only child doesnt exempt you from deployments.

That exemption went out after Vietnam. It was a draft thing anyway.

Get a little adventure while working one weekend a month, two weeks out of the year and never deploying? Please tell your friend not to consider joining .