My experience is that if you even have the faintest thought that the individual needs to go, trust your gut. Start making provisions to replace them. I say this because every time I have been there, I didn’t, and I was caught with a hole in my organization because I fired them hard for something else soon thereafter.
Gold. If you can't thrust 100%.
Also, over the years, when someone has given me a 2 week notice, I cut them a check and show them the door right then and there. I send them off with my sincere best wishes. If they no longer want to be there, I don't want to prolong the pain of them leaving. It's kinder to them and to me. Life goes on with or without them...
Can be difficult in some situations, but it's a good principle.