Sometimes, a person needs a decent swear, and this is especially true for those who are in the hottest of the hot spots right now, like Afghanistan. He used the F-word, okay, however, he didn't go into other very very shocking territories like what he wants to do to people, and how he wants to do it, etc etc.
Trust me, the F-word is candy compared to half the stuff that comes out of our military folks mouths, but you know what? Putting your life on the line constantly and seeing half of the stuff they do is bound to cause stress, as PPs have said. Personally, I don't think the parents of this soldier "get it". They could have sent him a PM asking him to talk to him about the stress, but you do not choose to publicly "discipline" someone who is in the situation this soldier is in, and quite frankly, his father writing that put up a barrier between himself and his son. The father demonstrated an inability to understand or to really try to understand the position the son/soldier is in, and because of that, his own child may never feel that he can truly divulge his stress/fear/etc. to his own father.
And see now I've got to stop writing because I'm getting all sorts of angry for OP's cousin myself