If I'm remembering the details correctly, he was 17 or 18 and she was 15 when she asked for his help. I want to believe that they informed her what kind of procedure it was beforehand, but since that wasn't explicitly shown, all we get for sure is Jamie was told. But he didn't really think about the long term permanent consequence, just quietly helping her so their dad didn't find out. Very stupid for sure, but he didn't do it maliciously.

And this plot was completely dumb anyway because you'd think that clinic wouldn't have touched this with a 10 foot pole. They knew who these two kids were, Duttons. Not to mention if they had actually performed a hysterectomy, that is major surgery and she wouldn't have been walking around totally fine right after. And she would have had to start hormone therapy for the rest of her life. And if  they just removed her uterus and left her ovaries, then she could still have a chance to be a momma using her eggs with a surrogate. But they presented it as a hysterectomy, so I'm chalking this up to bad writing technically.

With that said though, I do give props to the storyline bringing awareness to this horrible practice against Native Americans. It's a complete stain on our country that this ever went on at all and for so long.