Lol, and that's saying something. But seriously, that statement is utterly stupid. I have never, ever heard anyone say they told their employer they were entertaining another job offer.
This. It's all about the relationship you have with your upper management/owner. I've had some jobs where I told them I was looking/being approached, and others where I didn't. I always hope to have that level of trust and communication with my teams. If I'm open and honest enough with them and they with me then that's a great working relationship. I also always encourage them to look for the best opportunity and to let me know if that's now with me/us. I'll always support a person doing what's best for them and their family.I encourage mine to do so (and many do).. beats the hell out of being blindsided. But in general yeah, that's a great way to get shown the door.