Unfortunately, totalitarian and oppressive governments have the ability to do whatever they want to their citizens. Expats with relatives living in an unfriendly totalitarian nation are vulnerable to state sponsored extortion. Do you want someone that has a high potential to be exploited in a senior government position? This is an example of the least sinister reason why you would fail a security check. For 1 or 2 years after my divorce, I couldn’t get NATO security clearance because people in tenuous financial situations are vulnerable to exploitation. It’s a pretty simple concept. With security clearance any risk means you get denied.
Why are you assuming they agreed to be on the same plane? Nobody is that stupid and naive.