FFS, seriously.
OK, C1, immediate danger to life, possibility of direct contact with live energised parts, e.g. (NOT exhaustive) missing blank in a CU, you can stick your finger in, no matter how dumb, and touch a live part, busbar finger and kill yourself.
C2, a dangerous situation that requires 2 things to go wrong to cause a dangerous situation.
Again, not exhaustive, e.g. no earthing conductor on a property. Thus the EFLI is excessive and ADS will not occur, as there is no direct earth.
However, for this to be lethal, another fault (that's 2 in total) must occur for it to be lethal or dangerous.
When the install has no faults, there is no immediate danger, or hazard that could kill or injure.
C3, not compliant and should be improved, e.g. earthing conductor too small, no labelling etc. No real and present danger in the event of a fault, but, could do with bringing up to current standards to ensure that the installation remains totally safe.
So, what is the Ze for the TT install, does the fault current require an earthing conductor greater than 6mm sq?
If not, it will probably be cheaper to do the calc, prove it, do a test to confirm and give the client paperwork to show the original report is rubbish and go from there.
I would add to that report something along the lines of my opening paragraphs, and advise the client that they use competent contractors as required under statute law, and that there is nothing wring with the installation, but, the work that you have done in proving so, is cheaper than doing the physical work, and give them something to say that this is so.
Muppets like this must be discredited.