actually I don't think he said anything wrong, I think the way he worded it makes it sound worse
think about if he had said... "she's gonna be really pretty when she grows up" or "she's gonna be a beautiful young woman some day" see that doesn't sound bad
actually my boy said the EXACT same shit about my 16 year old cousin...
he said "you're cousin is gonna be bad when she gets older"
for a second I got mad, like wtf... she's only 16... but he explained he didn't mean anything bad by that... just saying she's gonna be pretty she grows up... I thought about it and realized it really wasn't bad... just the way he said it sounded like it