Well I´m pretty sure they will prevent him from boarding in FRA as it´s well known in FRA that Shenzhen is no longer a transit city.
The chinese airlines mention this on their websites when you wanna book a flight (but just in german).
Thanks for your thoughts, I will need to double check with the visa application center. I tried to find the mention on the website, but I couldn't,
It does only show once you selected the flights in the booking flow that "this itinerary does not qualify for 72 hours transit visa exemption", which is obvious since you transit through 2 different airports, but it doesn't tell you that it does not qualify for the 24 hours. If you know where I can find this info, feel free to share the link. The website does not give any transfer information at all for Changsha or Shenzhen, only for a couple of other airports, so I think it's quite unusual to transit there.
I will see what the visa application center says and also consult the airline and if all goes wrong ask them if they can reroute my via Changsha and CAN to SYD instead of via Changsha and Shenzhen, I have no high hopes, but better give it a try before getting a visa anyway...
This Shenzhen exception is really strange, especially that now also Air China is serving Shenzhen directly from FRA, and I can imagine that is the reason. Maybe the want to avoid that people then leave the country to HK by travelling to Shenzhen without visa or so...