Of course Hunter can be a witness.
If Trump’s team can demonstrate that there is reasonable suspicion (not proof) that Hunter was involved in criminal activity with Bursima, in effect indicting him, then that provides more than enough standing for Trump to initiate measures to persuade Ukraine to investigate Hunter - regardless of whether, and actually in the national interest because, Joe Biden is running for President. This process would need only meet grand jury standards, not legal conviction.
I would call Joe Biden and question him on his call to the Ukraine. Why did he put pressure to stop the investigation of Bursima.
Get the transcripts of the call. Get witnesses who were there for the call.