if this guy was half decent and had any sense of responsibility he wouldn't be fooling around with a 19 year old girl, especially bearing in mind that she's around his daughters' ages (wouldn't that weigh on him as a parent?)
But there's a genuine possibility that they're both on the same level despite the age difference and actually like each other (some people don't grow up, they just grow old); those kinds of pairings are unusual but do sometimes happen (they're just not socially acceptable anymore).
Also it may not last because whilst they might have fun together it could be impractical in the long term (can you imagine them still together in 10 year's time? She'd have to tell all her family first..)
Then again I don't know what kind of person she is- it's possible that they really are serious, and it helps to be open minded about the diversity of human relationships. Who's to say (apart from society but they may be wrong) that you can only love someone within a certain age range?
Having attempted an objective stance towards their relationship I admit it's harder when it's your friend that you're close to and that you may think of being similar to you- maybe you thought she'd never be that kind of person.
I'd also find the 43 year age gap quite daunting, but anything's possible. There's also the yuck factor, and the whole taboo of daddy issues surrounding their relationship (which is not exactly normal.)
They're probably not doing anything wrong, especially if she's confident like you say and able to stand up for herself, and they have been dating for a while. Keep in mind that she's a consenting adult and no longer a child so it's her responsibility and her choice. Talk to your other friends about it and your siblings, and get more opinions. If you really are worried that it's unhealthy you should approach one of her siblings first, the one that she's closest to, rather than her parents. Good luck, I hope that everything works out for you and your friend.