well, you're not the morality police and you're not her parent.
I realise that I'm just worried about my friend is that a problem? No need to get touchy.
I know a couple where the bloke was 22, was dating a girl since she 15. They are now happily married, with 3 kids and good friends of mine... i wouldnt have been very friendly with him when they first started dating, i know that for certain, but with the knowledge of hindsight, they are truly one of the most in love devoted couples ive ever witnessed.
My grandparents married when he was 22 and she was 17. And he is the best man I know. But that was 63 years ago.
I'd have no problem if they were adults. But this girl isn't, and she isn't mature enough for that.