Maximum
I defer of course to your superior knowledge (and, no, I am not being sarcastic)
But in my limited understanding of SOPs, they mostly have to do with technical parameters of the aircraft itself.
As a SLF, I would not mind in the least a Captain exercising his judgement to allow (apparently in this case a nervous PAX) into the cockpit.
If it helps the smooth conclusion of the flight, why not?
The Captain has a huge responsability, and must answer for errors of judgement which endagered the flight or other aircraft around the flight, but is it not a part of that same trust which must allow him or her to use discretion?