Registration of case in sub judice matter

First, let him come out of the case and get the facts recorded in existing proceedings.  Focus on coming out first, then he can think of retaliation legally to further get disturbed with mental peace.

There is no restriction in filing a cross FIR.

Cross FIR shall help the accused.

What is your concern/ locus standi ?

Let the accused seek professional services of a local prudent lawyer for better appreciaiton of facts, gudiance and proceeding, if there is some truth in the story.

Yes, it is usually the practice for the accused  to file counter case. Unfortunately, this practice (often prompted by the advocate for the accused) is leading to many an unhealthy situation compromising the administration of Justice.

There is no restriction to file complaint to register FIR at any stage. 

IF the trail not commenced for the original case then you can even file an application under Crpc 173(8)

and if trail started then you can file Crpc 156(3) application to register the cross FIR or can file a private compalint U/s 200.