If only the Supreme Court was an institution of law rather than a tool of corruption and ideological bias, there would be no question of how they would, and in fact must, rule.
The entire process of impeachment and removal from office is predicated on the idea that sitting presidents are immune from prosecution and ex-presidents are not.
It really is just that simple. The presumption right from the start, and in fact most of the actual point of the impeachment process, is that immunity only applies while the person is in office. As soon as they're out of office, they're "liable and subject to Indictment, Trial, Judgment and Punishment, according to Law." (US Constitution, Article I, Section 3).
But sadly, there's no telling how this wholly corrupted and compromised court will actually rule.