If they are suing United (and assuming press reports are correct - which is a big assumption), I don't see that they have a case. The published inspection interval for the fan blades was 5,000 cycles, and (reportedly) the failed blade had ~1,600 cycles since inspected. So United had done due diligence.
OTOH, if they are suing Pratt, they might have a case. Given this was the third fan blade failure is less than 3 years - and all three had presumably passed inspection - P&W certainly had plenty of evidence that either their inspection process was inadequate, or the inspection interval was too long (or both).
All that being said, I really don't think just being scared is grounds for significant damages. If it was, there are plenty of drivers out there that have scared the
out of me that I could be suing...