Well it is not a criminal case - since China has never claimed he is a wanted criminal - forget about convicting him, they have never even chargesheeted him.
Now imagine this
A Pakistani travels to Iran. The Nawaz Sharif government starts demanding his return but does not file an FIR, no chargesheet, no court summons, no conviction, nothing. Yet ever so often the FM / HM / DM issue statements asking Iranians to stop meeting that chap.
What else do you call this except political persecution?
Do you remember some time back when a Uighur leader (Dolkun Isa) settled in Germany was scheduled to visit India for a conference on Uighurs. He was granted an Indian visa - and then China protested [unsurprisingly] but this time they brought it to everyone's attention that the guy had a pending Interpol Red Corner notice against him? Since that was a legal problem his visa was ultimately cancelled. That is the legal way to go about it.