The problem is tht the boy is going to want to go faster than anyone would like. That just might make her uncomfortable enough to leave, if she isn't, then get worried, and test his motives, he will hint on his intentions even if he doesn't mean to. Your daughter is Los probably testing her boundaries with you, so make it obvious that you don't approve
It is not appropriate for the girl to date the guy because of their age difference and difference in life expectations. It will also be in appropriate for the guy to date the girl after he turns 18 years since some state don't allow it. At the age of 13 years, the girl is young and very vulnerable and needs guidance in life. As parents you can advise the girl to wait and turn 18 years to start dating.
As adults, maturity has leveled out a bit, leaving it okay for them to date. As adolescents, the brain has yet to develop fully and hormones control most of our actions. Seventeen is approximately the age that guys are at their sexual peak, so I would suggest NOT dating him when you're three years younger than him.