I'm Jamaican with waist length, super thick, lightly relaxed hair. The key to fitting my hair properly under the helmet is flattening out my roots (i.e. Blow dry and flat iron). Whenever I relax my hair and my roots are straighter, then it fits fine. As my roots get thicker, it feels extra snug. I also use a 100% silk cap to help flatten out my hair before I put on my helmet.
The idea that her only option is to cut her hair is laughable. That's like using a hammer to kill ants. It doesn't have to be such an extreme "solution." If she's willing to blow dry and flat iron her roots, she can get her hair to fit without compromising the efficacy of the helmet.
I think her issue isn't her hair so much as the shape of her head though.
BTW, I have 2 helmets - Troxel and Charles Owen.