I wanted to write that long-haul flights usually arrive well ahead of schedule but then I looked up this particular flight and it seems it has a really poor on time performance for some reason (not delayed only 65% of the time, average delay 28 ± 25 min):

https://www.flightstats.com/v2/fligh...ting/CI/15/SFO
https://www.flightaware.com/live/flight/CAL15

Still even in the worst-case scenario your friend should be able to make it to the high-speed train, which is the best way to travel if cost is not a factor. If the timing is really tight, take a taxi from the airport instead of waiting for the bus.

Should your friend's flight be really delayed, although I think it's quite unlikely, she can rebook to the 23:21 train, which terminates at Taizhong, stay there overnight, and continue the following day, perhaps on a regular train. Schedule:

???? Taiwan High Speed Rail (high-speed rail)
Taiwan Railways Timetable Information System (regular trains)

Note that the Tainan high-speed rail station is not really in the city. Arriving on a regular train, you will find yourself right in the city center.

I suggest to avoid long-distance buses for safety reasons.