Did they try putting the auto box back into drive, then back into neutral before trying the low range lever again?
It's actually easier with a manual as you can clutch/declutch with your foot whilst leaving slight pressure on the low range lever. In my old Defender, it needs this technique more times than it doesn't. I don't think anything is wrong, it's just allowing the gears to mesh before sliding into place.
I don't think the Owner's Manual is very expansive on this, just two lines:
"The two-speed transfer case selector has a locking ring that must be pulled upwards before a different position can be selected."
and
"To switch between the 'HIGH' and 'LOW', the vehicle needs to be stationary and the transmission must be in N."
Take a look at what goes on inside the transfer case for the high and low range gearing on the Grenadier...
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