@meri91364, I am very sorry that your friend's unresolved pain and lack of answers has him thinking that suicide is the only answer. Pain can lead to such desperate thinking. You must feel at a loss and want to help him.

Here are a few thoughts that may sound like trite and insurmountable at the same time when dealing with pain. But I sincerely hope they give you and your friend a starting point.
1. Contact the Office of Patient Experience at Mayo Clinic to understand why they couldn't help him.
Here's the contact info:
https://www.mayoclinic.org/about-mayo-clinic/patient-experience
Phone: 844-544-0036 (toll free)
Email: opx@mayo.edu

2. Call or text 988 the 988 Suicide & Crisis Lifeline (https://988lifeline.org/) to speak with a trained counsellor.

3. If you are worried that he may take his life, you can also call or text the 988 Suicide & Crisis Lifeline (https://988lifeline.org/) to speak with a trained counsellor or visit their website for helpful information of how to support your friend: https://988lifeline.org

Is your friend currently in the care of an AA specialist or neurologist?

So sorry to hear this. I read that there is hope ahead for this disease. Tell him to TRY and stay strong. Never take the answer of there is no hope. I know it’s very hard when your in this kind of pain. God be with him.