This is an area where the TSA will nearly always be one of the more pleasant experiences. By law, people in the US are protected from having to reveal private medical information to anyone, except under very rare circumstances. The TSA, for example, cannot demand a prescription or medical note for someone carrying medically necessary liquids or other paraphernalia. They also don't have the right to deny boarding. The airline has that say, and also is bound by the same laws the TSA is when it comes to access.