That's not new. Any recipe with mushrooms can be enhanced with the Death Cap, which is reported to be extremely tasty - except that it is also likely to be toxic about twelve hours later and fatal many days after an apparent initial recovery. It is believed responsible for 90% of fatal mushroom poisonings, including quite a few in which the timing was suspiciously convenient, and has been for centuries.
If you wait long enough for the Death Cap to take effect on the taster, you will only eat food that has sat around long enough for bacteria to multiply and for others to adulterate it after the tasting. I'm with you. I see no point in bothering with a taster, particularly for someone useless or malignant, such as the person this woman tasted for or others who may have a taster today.