There are a lot of checkpoints where this gas cylinder could have been stopped - checkin, security, boarding gate, and finally by the CC onboard. What is yet to be known though is if it were disguised or carried on board laisse faire (spelling?). If it were concealed then there is a strong chance the CC onboard and gate staff had no idea - and the pax may well have lied when asked the DG questions at checkin.
What I fail to understand though is how it was not picked up by security - my understanding is that gas cylinders of any form (excluding the small ones in the lifejackets) are not allowed in the cabin?
If it does turn out the pax lied when answering the DGs questions then definately there is a chance for recourse, however I agree the security firm is also at fault here.