I doubt if it is a small gap in their understanding that causes them not to see how they are hurting themselves and the people around them in an indirect albeit unconscious manner.
I think it is for this reason that the Buddha advised his followers to practise restraint from telling untruths and indirectly to cultivate an unwavering integrity within themselves. So that when someone point out our mistakes, we cannot hide behind excuses but are forced to face our real intentions. Sometimes, that can be painful as we are forced to unveil our concealed motives and acknowledge our hidden desires.
I find one of the tricks we learn as humans in order to make it easier to commit reckless, unethical and selfish actions is to hide our true intentions from ourselves and others. It is perhaps a reason why our parents get very upset when we first start to lie as young children. Sadly, I learned that we never truly stop lying even as adults; we just get more sophisticated with the process. We hide our intentions, obfuscate our desires, masquerade our selfish actions with layers of excuses and convoluted reasoning. Until, one day, our mind is totally ensnared in an entangled web of intrigue and confused. And finally, the original motivation for our selfish act is forgotten. (Incidentally, I think what the woman in the video really wants is attention. If the nail is removed, she won’t have any excuses to seek more attention and sympathies).
When to give advice? Perhaps, an appropriate time and situation but, I think, the most important factor is the person being advised. They should be people of integrity who are willing to look into their mind and heart. They should be willing to identify their mistakes, errors and problems with honesty. Otherwise, I feel we should just let them be. Meanwhile we can learn by observing the process of karma as it unfolds; especially develop insights into why doing things in a “wrong” way really is bad karma. (I believe, sadly, the woman in the video will eventually find herself all alone..... the very thing she wanted to avoid).
Observing how things unfolds with these people and where their lives lead will in turn strengthen our understanding of karma and the Dharma.