I think the thread is getting off track, there are two questions:-
1) Is the property potentially dangerous? If not then end of discussion.
2) If it is what is the spark required to do? (to watch his back)
The heading is rental with no RCD, and it has been said the new landlord law says it should comply to BS 7671:2018, but it does not say the installation should be treated as new, and as we start to read BS 7671 it clearly states the design date at which it comes into effect, so within BS 7671 it basic redirects one to an older edition. So following BS 7671 if installed before 2008 only supplies outside needed RCD protection, personally I think all homes should be RCD protected, but here what we are talking about is saying another electrician was wrong for not insisting on RCD protection, and I can't really say he was wrong.
If however you feel the electrician who said it was OK was in error, then what needs doing? And I would say a simple email, text, or letter to the owner is all that is required, it must be in writing a verbal is not good enough, but a text message is still in writing.
However we are looking at a watch my back get out of jail free card. So any message sent, could end up in a court of law, so one does need to ensure it is correct. So we are looking at
another electrician had done a report on the property saying it was all ok to start with and the fusebox was fine apart from being plastic not metal
and we have no idea if the another electrician was aware the property was to be rented. Or if it was a formal EICR or an off the cuff comment.
If I was to go around a property as I did here, I would comment it's OK to start with, but when you can you will want that consumer unit/fuse box changing for one giving RCD protection, it took me around 4 to 6 months to get around to it, it was not an urgent job compared with stopping the cistern falling through the ceiling. It could wait a while, as to being plastic not metal, I was not worried about that at all.
If a spark had written to me saying the fuse box must be changed, I would have considered he was trying to get extra work, and he would be removed from my list of tradesmen to use. A casual you will need to get that changed some time I would have accepted.
I know I considered renting out my mothers old house, and I got a letting agent to tell me what needed doing, I was rather surprised at what they wanted, the work top one side of kitchen was lowered, as mother in a wheel chair, they wanted that raised again, it had a domino hot plate, they wanted that swapped to one with four heat areas instead of two, and the list went on, including removing the shower in the bedroom, and enabling the shower in the bathroom. I had an offer to buy the house, so sold it instead.
Yes some valid items raised, there were sockets in the bedroom closer than 3 meters to the shower cubical. But not were they could really get wet, technically non compliant I know, but no real danger.
So it seemed the house needed to be in a better condition to rent, than required for an owner occupier to use, but doing an EICR we don't write on it for rental or owner occupier, it is the same for both, except the owner does not need to follow any suggestions if not renting.
There should be no code C1's as if that bad the inspector should make it safe before leaving.