PTSD is generally associated with personal suffering rather then as a third party, including bereavement. The most often understood examples started from US POWs who underwent bad treatment at the hands of the Viet-Cong but of course there were others before and after. PTSD almost always affects behavioral pattern in the aftermath of the event, which did not happen to Jack Thayer after he lost his father in 1912. He coped with the bereavement and got on with his life, but as I said before, it might have left a scar somewhere in the confines of his mind. Therefore, loss of his son 33 years later might have acted as a 'booster' to bring about the suicidal intent and deed. His personal sorrow undoubtedly led to the suicide but IMO that is not the same as PTSD.
You also have to remember that parents and other family of young people going off to fight in any war always have the fear/knowledge in the backs of their minds that their loved ones may not come back. Therefore, it is not as unexpected as losing a son or daughter in an accident, for example.
I have to disagree with that opinion. Fleet's experience on board the Titanic during its maiden voyage was not the sort of trauma that could have led to PTSD. He was one of the lookouts at the time, yes, but he did his duty properly as far as is known and he certainly would not have carried any guilt complex afterwards; sorrow for the loss of so many lives including colleagues, yes, but not guilt. He was rescued on an early lifeboat (#6) and so the experience on board the Titanic per se IMO had noting to do with his suicide 53 years later.
On the other hand, his childhood experience of being abandoned by his mother, growing up in foster care etc would have left a major scar. That would have come back to haunt him in the 1964 when his wife died and her brother evicted Fred; that sort of scenario is just the sort of thing to aggravate latent suicidal intent.