I am no expert either, but according to my friends who hold PRC passports they have no problem transiting TPE with one exception: departing from the mainland and transiting in TPE. They say that they may be asked for a current Taiwan travel card (for individual) by Chinese immigration at the departure airport. No one is clear what would happen if you don't have it.

The card is issued by Chinese authorities and they have not been normally processing applications on the mainland since 2019, maybe 2018, when China banned individual travel to Taiwan. They do readily issue them in overseas consulates for overseas residents.

So before leaving the US, to be safe ask your friend to check with their nearest PRC consulate to see if they need a Taiwan travel card to enter or exit China on a flight to or from TPE. Don't wait until arriving in China.