Shane's letter claims he was in the hospital- and that his reaction was so severe he would be badly brain damaged. Being how much Shane loved Monty- I find it hard to believe he wouldn't speak out if there was a chance Monty would recover.
The first stage of grief is denial. It's the time you take 1% chances- even if the outcome is just as shit.

The alternative is that because he was exceptionally kind to Sheena as well- he could have been blinded by her emotions, and "let" her pull the plug (even though legally- shitty as the law is- Sheena had every legal right).