https://pcpartpicker.com/list/VgdDDqFauna should get this build, or something very similar to it.
Put the processor into 65W Eco mode. Almost zero performance loss in gaming scenarios -- but significantly less power used. Which means less heat generated. Which means the fans don't have to work as hard. Which means the computer can run almost silent.
4080 chosen for similar reasons -- yeah, it's a bit shit for most people, but it's super energy efficient and runs very cool. Which means you can crank the fan down and have a silent monster of a GPU, perfect for a streamer.
Case has giant wind turbine fans, again just set them to minimum speed for minimum noise. This case does get super noisy if you let it ramp to 100% fan speed, but that's what fan curves are for.
Really just aiming for high performance without being overkill, plus absolutely overkill cooling solutions so she can ratchet all the fans down to 25% hard limit and still be fine.
