BBforks hits a very good point about the copper line and safety-liability.
I doubt the copper line is rated to LPG high pressures, more likely it is rated to household plumbing pressures and for potable water.
Change it for the proper steel tube or LPG rated flex hose ASAP. Park the truck in the mean time. They/You won't see any LPG bubbles leakage with soapy water, on that line, unless there is a vac. on the VFF30.
Tell them they need training in LPG b4 they hurt someone, not to be mean, but this is exactly why they need the training first.
An LPG fuel engine fire is almost never fun, even if it is exciting. The amount of fuel that spurts out of that hose in 1 blast could easily envelope the entire forklift in flame, with only a missing spark plug fire to ignite it.