Being 34 and dating a 19yo is some weirdo shit. Your friend is engaged in a weirdo situation. Mostly bc he’s probably robbing her of the experience of being a 19yo. The age dynamic also belies a power dynamic and this feels eerily close to a grooming situation.

But it’s legal.

You can disagree on moral grounds, but he’s not doing anything inherently wrong. He’s just doing something you disagree with (and something I’d also have a problem with).

Maintain your friendship knowing he’s doing weirdo shit or just fall back and wash your hands of the situation. Those are your only choices.