I was always taught that good decision making comes from gathering all the information available, assessing the options open to you and then deciding on the most appropriate option based around the situation you face. If you can't comply with the rules then your decision is made for you.
Unfortunately, your decisions may subsequently be disected by people with greater knoewledge and experience, under no time pressure and with many more resources available to them, so you had better bear this in mind when assessing the risks and coming to your conclusions.
Braking action measurement may well be flawed, but it is the best info I can get regarding the runway state and the associated risks should I need to reject. A ride in a car may well allow me to have a closer look, get a feel for what the car behaves like under braking etc, but it is not an exact science, so I will stick to my company procedures, which comply with JAR-OPS and request a braking action report. If that throws any doubt upon the suitability of the surface then I won't be going anywhere until such time as something has changed for the better.
I always think how I can justify my decisions in a court of law (should I be lucky enough to survive) and that sometimes helps with the go/no-go decision. I am lucky to work for an employer that genuinely says "if you are in any doubt, DON'T DO IT".
Another point is that in the UK we are not as experienced at true Winter Ops. Luckily we have a sister airline in the group based in Scandinavia, where obviously Winter Ops is their forte and we have been briefed and amended some of our SOPs based on their greater experience. This captain may well be Scandinavian for all we know (there are a lot of them about in UK/ Irish aviation these days!) and may well have much more experience than the likes of cautious old me.
The main thing is KEEP IT SAFE, within the rules and your own limitations, your crew limitations and any other factors which influence your ability to do so. There has been no evidence thus far that has convinced me that this fellow professional hadn't done just that.
PP