The fact that your friend can ask you for a favor in return is Guanxi, and is certainly a characteristic of Chinese culture that provides accountability and also is factor in whom one befriends.

The fact that a favor was required to get a visa that presumably you qualified for (or if it was used to get one that you didn't qualify for as well) is borderline corruption, even if no currency or goods were exchanged, since Guanxi dictates that at very least a favor is now owed. I have friends from China that talk about situations like this that include obtaining marriage licenses and dealing with fees associated with second and third children.