That's a actually apt. You don't know if they tried prosthetics, if they have prosthetics, or if prosthetics would even help. You're not a doctor, and you're not their doctor, and you say "just 3d print a new leg" because you saw someone do that in a new article.

Wearing a prosthetic leg can be taxing, and painful, and destabilizing. It requires a whole apparatus and special exercises, and balancing on two would likely require crutches as well.

Most of all, the person you're talking to probably knows more about their condition and their options than you do. So saying "just get prosthetics" is not a helpful suggestion at all.

Someone with ADHD who rejects your "helpful" advice isn't choosing to remain unhelped. They're saying you're an asshole for thinking you're the first person to come up with "just keep a calandar. That's what works for me."