It will go to SCOTUS, the constitution is clear in its language and doesn't say anything about trying a private citizen for an impeachable offence.
Article 1 Section 3 Clauses 6&7
The Senate shall have the sole power to try all impeachments. When sitting for that purpose, they shall be on oath or affirmation. When the President of the United States is tried, the Chief Justice shall preside: And no person shall be convicted without the concurrence of two thirds of the members present.
Judgment in cases of impeachment shall not extend further than to removal from office, and disqualification to hold and enjoy any office of honor, trust or profit under the United States: but the party convicted shall nevertheless be liable and subject to indictment, trial, judgment and punishment, according to law.
Now he could still be indicted by the feds for inciting insurrection and sedition but I seriously doubt they will