I am not aware of any legal minimum specified depth, however 522.8.10 in BS7671:2018 simply says "Buried cables, conduits and ducts, shall be at a sufficient depth to avoid being damaged by any reasonable forceable disturbance of the ground". So under a concrete path can be shallower than say under vegetable plot.
Whatever he does, I'd use armoured cable. I still have mine connected waiting for a new shed, but looking at it its completely unscathed and that would not be the case with anything else out there for that long even if it were in some conduit.
Brian
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