As the claimant clearly does not need the money since he has substantial income and the poor lady killed had considerable life insurance, I can only presume the intention of taking legal action is to put the airlines concerned on trial.
AA and UA will then be forced to prove to the court and jurors that they took reasonable action at the departure airports to prevent known terrorists boarding and taking knives on-board etc. As the risk of hijack of civil aircraft is entirely foreseeable the airlines will need to show they had taken the risk seriously and had adopted appropriate security measures to prevent it happening. This will surely be the focus of the case.
The novel issue with 9/11 is that the aircraft themselves were subsequently used as weapons - prior to 9/11 it could be argued that this was not foreseeable (although in fact some imaginative writers and film makers had foreseen this as a possible terrorist plot for a book/film several years previously). However had the original hijack been prevented, then the twin towers could never have been attacked in this manner and the lady concerned would not have died. Whether or not the attack on the World Trade Center was foreseeable is not the issue - the potential for hijack was completely foreseeable and the airlines will need to prove they had adequately defended against it. That will be very tough -since they clearly they did not adequately defend against it.
During the case the facts of the tragedy will be tested to see if the airlines were or were not culpable, and if so to what degree. But what about those other organisations who should share some of the liability?
It might be interesting to see who else AA and UA implicate during the trial, in their defence. What did the FBI do to apprehend these known terrorists before they could undertake their attack? Why did US Immigration allow them into the country in the first place? How come several of the terrorists had been freely able to move in and out of the UK and across parts of Europe during the planning and preparation for the attack and why were the UK Government and EU Authorities so lax?
I agree the motive for taking legal action is doubtful but some of the above questions require answers.