What’s your opinion on a lay person carrying a knife? What if they have basic training?

Carrying any weapon increases the statistical likelihood that you will die to a weapon. Owning certain weapons increase the statistical likelihood that you die to that weapon. This is because if you bring a weapon to a fight, the other person often responds in kind and being armed when someone else is armed means tensions are higher and they are more likely to use deadly force. In the worse case scenario, you lose your weapon to someone else and get killed by it. Training will reduce the likelihood of being disarmed and increase your proficiency with any weapon, but what @TexMexBazooka@lemm.ee has been stating with regards to anyone being a deadly threat is extremely true- real confrontations don't happen like they do in movies and even a highly trained individual can be killed by a complete novice.

If you're looking for a self defense weapon one of the best self defense weapons that exists is not deadly at all, but rather debilitating - mace/pepper spray. It requires no training to use, it can be activated very quickly, and it is very portable. Depending on where you live it is possible that it is illegal, in which case this is simply not an option, but in most countries in the world it is a legal form of self defense. In countries where it is illegal, there are usually other irritant sprays which you have access to which may not be as strong but will generally speaking do the same thing. Some sprays are also designed to be slippery in addition to irritants which can make holding weapons difficult.