Wow. I almost said "things were so different when we were young." But the bottom line is, they weren't.
I was not quite 16 when I started dating my almost 21 year old exBF. He was a good guy in general, never disrespected me, never pushed me for sex until I was ready. We dated for almost 3 years, and he didn't use me & leave me or anything like that. My parents thought he was 17 at first, and his age crept up slowly every few months, although I don't know if they ever realized just how much older he was. He was my first true love.
BUT. Looking back, I realize that he liked being in control. Maybe that's not the best term to use, because I've always been a very strong person, he didn't "control" me really. Yet I remember things he would say when we'd argue, and the way he'd say them - he sometimes acted as if he were an authority figure to me. He liked to think of himself as teaching me things about love & life...ew, I feel nauseous, it's so icky to me now. Like, I told him to f*ck off once, and he basically lectured me & told me "don't ever speak to me that way again!" I almost expected him to add "young lady."
Anyway, when I was 19, he proposed marriage.

Something inside my head snapped/woke up & I realized that this is NOT what I wanted for my life. The breakup was bad, really bad, we don't speak at all. Not that I care to, I've honestly grown up & out of that part of my life. But if I were a more impressionable person, someone with lower self-esteem or more self doubt, I don't know if I would have gotten away & gotten a life.
So I guess the point of my telling my personal story that nobody cares about, is to be open with her & honest about why you think this is a bad idea. I wouldn't forbid her, but I wouldn't enable either (don't let me stay at your house hours on end, day after day, don't drive her to see him, etc.) And hope & pray she's strong enough on her own to not "need" a man - whether it be this guy or another.