Mr.Victor, the round-trip flight tickets are mandatory for China visa application which can prove your planned stay duration in China, I know that it is not reasonable because you have not gotten a China visa, you don't know how many day you are permitted to stay, however, it is the Chinese visa rule.

If you have an invitation letter from a qualified China Travel Agency, you don't need to provide the round-trip flight tickets.

In fact, the invitation letter from your friend is not helpful for your application because they are not your relatives, you'd better also provide the photocopy of your friend's passport and Chinese residence permit.