What will you do? (Ethical dilemma question)

[Mr.Obvious]

That's not an ethical dilemma though.
His GF is a sexual predator that is lying to him. It is your obligation to tell him, after perhaps as Losty pointed out, after you file a police report. There is no dilemma.
If he resents you for it when the truth comes out or he doesn't believe you at all, that sucks. But keeping those possibilities in mind doesn't make your choice of option 2 any more ethical. Instead its more selfish, actually. You don't want to upset the natural order and would rather let your friend get cheated on and have a sexual predator go on harrassing others? That's cowardice, not ethics.

Lol let's just erase the assault part because that's not doing this discussion any favor 😂

Question boils down to, would you rather your friend be ignorant but happy or informed but sad? I think you're leaning over to the informed but sad side? You do realize although you're being honest here you're still ruining the happiness of your best friend though, right?... What if he goes on a downward spiral and ends up taking his own life or something? Because we're talking about a 8 year+ relationship... It's safe to assume that shit will probably sting

Informed, with a chance of ditching an unwanted partner in favor of one that would actually love him as he deserves.
Shit will sting. Because of what the GF did.
Still no dilemma.

You can add all the hypothetical scenarios that "could" follow after.
He might go into a downward spiral and kill himself.
He might find someone better and become happier than ever.
He might find out 16 years down the road when the same thing happens with a new guy, making for an increased loss because now he's wasted another 8 years you could have prevented.

You can't tell the future. When you look at the here and now, there is no dilemma.