Alito is writing the decision. Roberts can simply add his own.
The fact that Alito is writing the decision means that Roberts likely is NOT in the majority, since the Chief Justice gets to appoint who writes decisions if he is joining them.
In the case of the ACA (Obamacare) decision, Roberts was the swing vote, so he got to write it either way. He desparately wanted a 6th vote, which he didn’t get.
One of the difficulties with Roe was that it was 7-2, with one vote swung in exchange for a cite of Pete Rose in an unrelated case, and Chief Justice Burger changing sides so that he could appoint the least competent Justice (Harry Blackmun) to write the decision.