This is how I see it (but I'm not a National Director so would welcome contribution from anyone else)

Clearly North has received Unauthorised Information that South was considering something other than Pass

According to Law 16B 1(a) (which is better to refer to than 73D) North may not select the option which was demonstrably suggested by the Unauthorised Information

As I see it neither bid or double is demonstrably suggested by the UI so any action by North is Ok here
South might have been considering double OR bid on

Furthermore many pairs play a pass in that situation is forcing ("I'm not sure whether we should bid on or double; you decide, partner")

Even if that is not an explicit agreement it is generally accepted if one side bids a game they expect to make and the other side  sacrifices then they should always either double or bid on.
To collect undertricks in 50s (or even 100s) will always be a poor result for a hand which can make game.

I'm not a National Director either or even a Tournament Director but I think Nick has this exactly right.

Don't worry Nick this probably won't happen again 

Hi Kevin

Long time no see, hope life is treating you well.

The vulnerability may matter (if they are red and we are green that might change the dynamics), but assuming euqal vul or us red them green...

I'm not a director either, but see it through the lense of a frequent appeals committee member.  If this came before me in an appeal context I don't think it would be a long meeting.

Nick sets it out well from a rules perspective, I'd only add that even if you were to assume the slow pass means they are better than an absolute minimum (not a safe assumtion as Nick explains) and hence I possess some information I am not entitled to, I don't see passing in 5S as one of the open chocies (to me it is not a "logical alternative" to a double).  Even if my partner is not the type to play forcing pass in this type of situation, I think the logical alternative to double, if there is one at all, is 6C.  Having said that, we are a blanaced near minumum for our choice to bid 5C, so moving on to 6C now seems a bit rich.

Best regards

Scott