Does anyone have advice on how "buddy letters" should be written for undocumented injuries (for getting a VA disability rating)?

No prescribed format that I know of. If an incident, a brief description i.e. To Whom It May Concern: I, PFC Long T. Schmuckatella, was present the when PFC Wheeler fell out of the window on the second deck of the barracks at Ft Jackson. He injured his leg but didn't report to sick bay because the platoon sergeant threatened to beat him to death if he missed morning formation. Wheeler had limited use of his leg from then on and could not run or dance as he could before the fall. I was with him for six months after the fall and his limitations were obvious throughout that time period.
Signature'

The more detail the better and it being notarized wouldn't hurt.

I wrote one for a shipmate once. He was an asshole, but I had knowledge of the incident, and it didn't seem right to let my dislike of him color that. It worked, he got the additional %.

PFC Jim Wheeler I just did one for a friend, who retired in 06-07. I think you should consult with a VSO (Veterans Service Officer) at your local service organizations (American Legion, DAV, VFW etc), or your state veterans commission. They call know how to the negotiate the bureaucracy of the VA.