How can not being diagnosed with a mental illness that a person has affect them? -example: person with adhd/depression never being diagnosed with adhd/depression

Mental illness that goes untreated can lead to serious emotional, behavioral, and physical health issues.

Explanation:

If they were never diagnosed, they could have never been proven in anyway to have that.

answer; eat a strict gluten-free diet;

tell their parents and make sure she gets from a counselor or therapist. be supportive and be there for her. be a kind and good friend. spend time with her to make her feel like she's not alone. i'm not a professional but i really hope they stays strong.