I never liked being asked in private if we should kick someone, it should be laid out to them openly who and why. After messing up a 3rd time the same mistake, it's not just a mistake, go over it with them and ask if they feel they are up to the task. After the 5th time with the same mistake, It might be best if they drop out for the time being, but if they feel they still want to try, I myself would be willing to give them time to work things out, but ultimately it would be the decision of the group if they wish to offer the time to help teach their teammate. Not pushing guilt to ether side as I know people only have so much time to game or not the patience for teaching as they have their own goals for their session, but on the other hand, players will have to eventually need to learn the mechanics, somethings might take a bit of time for some. It took me two runs to learn how to fight the giant book monster in library.

If you will hear my 2 cents on it, be open and patient when you can so as to not alienate or frustrate other players, especially if they need to be replaced for a run. Be polite and open about it.