From official guide (in french i will try to translate in english)
Even if nothing show it on screen, buddys except Dwalker have both a health and Stamina stat (who change depending of a points system hiden too)
The value of their health and stamina decrease when they take damages from lethal and non lethal weapons respectively.
If they lost the totality of their health or stamina they fall KO and are repatriate on Mother Base and stay unavailable for some time.
Hope you can understand everything with my bad english haha
[Edited by djodius, 9/1/2015 6:53:31 AM]
CAN confirm. I have the English official game guide.