Marco Polo, I know you said you weren’t the right person in Sharon’s life to completely set up a therapy appointment. But speaking from personal experience it might be enough to just firmly tell her you take her problem seriously and she should see a counselor. This is just honesty. I feel it is a more useful type of honesty than telling her she should avoid patterns of negative thinking.
Kind of like if she had some other troubling symptom, as a good friend you would flat out say “you should see a doctor about that” rather than saying “you should try to heal that yourself.”
This response and most of the responses in the thread assume she hasn’t sought treatment yet.