Amateur diagnosis: a dilemma

I saw a woman at Starbucks earlier today who almost certainly has Cushing's Syndrome, a rare-ish debilitating disease that is curable with treatment. It has some very clear physical signs, like a "moon face", a hump on the back, and central obesity with normal sized extremities. The woman I saw might not know she has it; thousands of people live in misery with undiagnosed Cushing's for years. It can be fatal.

What is the right thing to do in a situation like this? Do you tap her on the shoulder and kindly tell her your amateur diagnosis, possibly saving her life, or do you let her go on her way, potentially living in misery?

I stared at her for about 10 seconds, thinking about what to do, and then left without saying anything.