I dont have experience with the FX range, but have a lot of experience with the 06 and 07 range. Not the same, but similar enough.
The aquastop valves on the 06 range are fiddly when you are trying to reattach them to the motor head IME, and if the valve and hoses arent lined up perfectly with no twisting the lever doesnt want to pull down and lock in place. I found a new valve sorted the issue for a couple of years, and then the problem reoccured, so presume its an issue with wear and tear. The 07 range seem a lot easier to reattach than the 06 range thus far.
I've broken 2 aquastop valves so far using too much force when they didn't want to reattach, and the last time I broke one a piece got stuck in the motor head, I couldn't get it out and ended up replacing the whole filter rather than just a new aquastop valve.
Did your neighbour manage to get the valve back on?