As stated, it depends on what the conviction was for. I can tell you from experience, I am a convictd felon under the Adam Walsh Act for an offense that happened 10 years ago. My wife and I have been trying to get a petition approval for more than 5 years, but have been unable to do so because of my conviction, despite the fact that I have sent a huge amount of evidence to try to overcome their denials. Our case has been denied 2 times and I filed an appeal with the Board of Immigration Appeals.
In regard to your friend getting a Chinese visa being a convicted felon, I don't think there would be a problem with that. I have received one 4 times after disclosing my conviction to the Consulate. In fact, the funny thing is that for the previous 3 visas I got I didn't even have to go to the Consulate to get it. I had a visa service company do it for me. They knew about my conviction and never even wanted to interview me. But, this recent time I put on the visa application that my occupation was Broadcasting/Journalism. THEN....they wanted me to come in for an interview with the Consulate Officer before they would gve me a visa.
So, your friend will have no trouble going to China and meeting someone. However, if the offense was anything related to the Adam Walsh Act, unless he is willing to go through a lot of pain and hardship and put her through it too, or willng to spend the rest of their lives together in China, then tell your friend not to waste his time. He should just find someone who already lives in the US.