Chinese Passport
 

I believe your friend will be in trouble when exiting China since he could NOT prove that he was legally admitted into China using EU passport. At that time, if and when your friend shows his Chinese passport as well as the EU passport, his Chinese passport will be confiscated for sure.

It is NOT wise to play this kind of the game in my opinion.

I don't buy this argument because people with Chinese passports don't need to prove they've been admitted to China.

I think the point is, should the Chinese authorities find out about his EU citizenship while he is in China, he could be detained for entering the country without a visa and get into major trouble. Having a passport isn't the same as having a citizenship. I don't know how exactly this is handled in China, but if the Chinese citizenship is automatically lost when aquiring another citizenship, it means he was no longer a citizen when entering China - regardless whether his passport was still in his possession.

Of course, if he is lucky, it will probably work. But there is a risk involved.

Yes, the airlines want to make sure that you are not returned on their dime.
So once they issue a boarding pass exit officials just verify identity.

I thought overseas Chinese are ethnic Chinese who don't have direct ties to the PRC (or the RoC/Taiwan).

I still fail to compute any of the luck/risk at stake, assuming the actor isn't stupid enough to produce both documents at the same time. In other words, the exit guy isn't empowered to perform any crazy searches; show him the Chinese passport and move on.

The more I've thought about the OP's friend's case (during the evolution of this thread), the more I've come around to the idea that the original plan is/was solid.

As part of the airline contract with the government , the airline bares all the financial liabilities associated with denied entry to a foreign country.

13.3 Refusal of Entry

If you are denied entry into any country, you will be responsible to pay any fine or charge assessed against us by the Government concerned and for the cost of transporting you from that country. The fare collected for carriage to the point of refusal or denied entry will not be refunded by us.

13.4 Passenger Responsible for Fines, Detention Costs, Etc.

If we are required to pay any fine or penalty or to incur any expenditure by reason of your failure to comply with laws, regulations, orders, demands or other travel requirements of the countries concerned or to produce the required documents, you shall reimburse us on demand any amount so paid or expenditure so incurred.

OP wants to enter mainland China with PRC passport and leave with EU passport. The risk is that exit control will check his entry stamp on the EU passport. No entry stamp no exit.

If the OP uses his PRC passport to exit, he needs a valid visa. (Yes there are a few countries mainland Chinese can go without visa, but somehow the exit control guys don't seem to care)

I'm not familiar with the rules governing HK/TW id. I assume it's much easier as you basically have dual citizenship with two entities other than PRC.

Again, the concept is to show the airline the foreign passport and the exit official the PRC passport.

Chinese Passport
 

I would suggest that if your truly believe that your friend's method is sound and workable. He should then proceed with it and report back to this back here after he returns back to EU.

Good luck to him...

Not my friend, the OP's. In any event, the fact that many people work it on a regular basis (including my own best friend, albeit with HKID) suggests that it's tried and true.

One hitch could be that some airlines have started printing the passport number on the boarding pass.