As a general rule, I'd say when you have to resort to ACKSHUALLY'ing with age of consent laws the battle is already lost and you should probably shut the fuck up sooner rather than later.
I find it kind of sus the chick waited 4 years to drop these DMs if she thought he was such a threat, so I'm guessing this is more about upping her profile than outing a predator, but regardless of her motivations there is absolutely no context where these DMs are not at best inappropriate and at worst creepy as fuck.
There is absolutely no reason on this earth for a man in his 40s to be talking to a 17-year-old like this. Actually, her being underage isn't really relevant: even if she was 18-21 this would still be supremely creepy. If he just wanted to praise her work then why couldn't he leave a public comment on her posts? Dude knew exactly what he was doing, fuck this piece of shit.
ETA: God, he's even got the groomer speech patterns. "I feel like you'd be a good partner in crime haha. You're not a snitch right haha? You're a good girl haha."