Adolescent Daughter Involved With an Older Man

Should we let our 16-year-old girl spend time with an older guy? He's 25 and very involved in our church praise band. For the last year he and my daughter have had a fun brother-sister type relationship. He's very likeable and held in high esteem at our church, but for some reason my mother's intuition has kept me on the alert. It's not that I don't trust him – it would be entirely different if they were the same age. Besides, my daughter constantly assures me that they are just friends. Should I be concerned?