I doubt whether anyone can prove that General Electric or Boeing were at fault. It has been suggested that certain parties knew the condition of the engine, but this is pure speculation.
Comments in one of the other threads about this incident suggest that there was tension between the FAA and GE/Boeing. There was disagreement as to what the maintenance / inspection regime should be following the discovery of cracks in a compressor disk in this engine type.
The FAA, to their credit, imposed a more onerous and expensive regime than the cheaper one advocated by Boeing and GE.
No doubt the report into the incident will cast light on whether the FAA mandated regime was rigorous enough given the evidence available at the time. I can't imagine it will praise Boeing / GE's original stance on how the defect should be handled. However it's perfectly possible that, given the evidence available at the time, that their position was not unreasonable and that the FAA was simply playing it safe.
We won't know until the report is out.
Since When Did a Court Base an Engineering Decision on Reality?
And there in lies the problem for Boeing / GE. Now that this incident has happened it will be easy in court to portray the sequence of exchanges between Boeing/GE and the FAA unfavourably. If the accident report is damning of one or the other, then arguably they'd deserve what would be coming to them.
Cooperate With the Regulator, They Are Your Friend!
There's only one real way for a company to defend itself against actions like this. A company can reach cooperative agreements with the regulator. The stronger the regulator, the better. Then in the event of an incident there can be no question of malice or negligence. That will automatically reduce any court awarded damages, or possibly eliminate them altogether.
If Boeing or GE have spent the last few years trying to reduce the authority of the FAA, they may be about to suffer the consequences. Lobbying for a reduction in regulatory oversight is a good recipe for short term gain for long term pain.
Uber are making the same mistake in the taxi market. The taxi regulations are there to protect customers
and operators. If a firm has done everything required of them by the regulators, it is hardly their fault if a rogue driver ends up attacking someone. By ignoring the regulators they are taking on full responsibility for all consequences, with an added dose of wilful negligence thrown in for good measure.