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If you look at the criteria for mental health disorders, you will see that, in both the 70% and 100% criteria, it lists "Suicidal ideations".
Scroll down to 100% and 70% respectively:

(The 100% category does not call it SI, they call it "persistent danger of hurting self or others". Or, abbreviated SI HI)

http://www.vva.org/ptsd_levels.html

It is unclear to me how the VA can propose to reduce on the basis of meeting the criteria?

Based on this, it would appear to be error on the VA's part. You do need to request a hearing, and, probably a lawyer.

If it was me I would have her write a statement and have her go with you to any review or c&p exam. Present the statement to whoever is providing the review and submit the statement to whatever Regional office or whoever has asked for a review. Normally it is the regional office who sets up the c&p exam. If and when you mail it make sure that they sign something that shows that they received it. In the United States it is usually a certified letter and the mail people return a green card showing a signature of who received it. Keep a copy of the stuff you submitted and staple or attach the original statement and keep it for your records.

Yes, you can use a THai GF buddy letter.  Buddy letters do work, provided they are consistent with medical evidence.  The CAVC says that they can not "dismiss" your evidence based solely on the fact that you  (or her, if you are supporting her) are an "interested party".  Its called "lay" evidence.