Will an EU residency card be sufficient to depart China?

I have seen the claim that Chinese citizens must have a visa for the destination country before leaving China before - it was, of all places, posted on a Wikipedia talk page.

I can state categorically from first hand experience that it is not true. Last March, my family, who are PRC nationals, and I, British, went to Korea as part of a cruise. Because we were part of the cruise party and entering Korea via Jiju Island, Korea waives the visa requirement. There was no visa in my family's passports when we exited China. The official checking the passports did not look for a visa.

When travelling to the UK three years ago, we did have visas in the passports for the UK but were flying via Warsaw which allows visa free transit. The official checking documents for exit from China did not look at any page other than the photo page of the passport for my wife and daughter.

Thus rumours that PRC citizens must have a visa in their passport to exit the country are incorrect or more probably out of date. The airline will check that you have suitable documentation to go to the destination when you check in. The entry/exit official does not, in my experience, check anything beyond what you would normally expect.

Quick answer: My wife's Chinese, she holds a Swiss residence permit and she has been back to China twice since receiving it. The EU residence permit is definitely enough.

She could not even obtain a visa for the EU since she's holding a residence permit, therefore that's the only option she has.

Your wife certainly won't be the first EU resident to depart from China. There are millions of people worldwide holding EEA residence permits and at least 350 thousand of them are Chinese citizens. The permits are well known by airline employees since they are fairly standardised across the participating countries:

You won't have any problems departing for your destination.

Source: done it myself numerous times.