As a guy who watched lots of the girls his age at college start to date guys 5+ years older than themselves I remember feeling jealous at the great insight they were able to get from those experiences. Children with older brothers or sisters are usually much more sensible and grown up than those without, and the same goes for girls who date older men. It's probably just a sign that she is highly intelligent and mature for her age anyway.

Women mature much quicker than men and by dating up in this way they continue to surround themselves with much more mature and sensible people.

It totally depends on the character of this person - which by the sounds of it is good - but he may be a really good influence on her. Far better than dating a guy her own age. Do you remember what you were like at 17? Weren't boys at that age more likely to be 'only after one thing?' It's no wonder women aren't interested in men their own age. Teenage boys have literally nothing of value to offer anyone.

Also anything you do say or do will only make the situation bad between you and her. If he actually mistreats her or starts seeming like a bad influence then sure jump in there and say something, but otherwise you are probably worrying needlessly and causing undue drama.

My 2 cents.