Yosef Karo, Translation:Shulchan Aruch — Orach Chaim 594…
“ However, if an individual person [not in a minyan] has not prayed the nine blessings [of the Amidah] , somebody else cannot pray to fulfill his obligation. Rem"a: Some disupte this and say that if the other person can pray, he can fulfill his obligation to prayer (Ra"n at the end of Rosh Hashanah, Mordechai, and Hagahot Asheri in the first chapter of Rosh Hashanah) . ”
