In normal conversational English, you would shorten this to "She's going to" or even "She's gonna". But one way to emphasize a phrase in English is to say each word of the phrase clearly and exactly, without shortening any of it. So this phrase is pronounced with a slow and regular speed, and stress on each word of the phrase:
She - is - going - to - flip - OUT when she hears about this.
This expresses more emotion than just saying the sentence with normal conversational speed and stress:
She's gonna flip out when she hears about this.