framer - I believe from previous posts that b...b is with BA and has limited exposure to the airline world. I know from chats with BA guys down route that that indeed is their SOP, based on the F/O being 'P1 qualified' by BA during training.
As you say, his comment is only BA specific and there is no general airline-wide 'SOP' for the conduct of walk-rounds.
For me, I interpret 'the rules' to be that I have to be 'confident' that my F/O is capable of correctly conducting a walk-round and that is the basis on which I sign the tech log as 'fit for flight' in that respect. If I had doubts, I would do it, yes, rain or no rain.....