The ruling on one who prays a rak'ah without an excuse, then a ma'mum joins him

Question : Nothing. If one prays a shortened prayer of one rak'ah and a man comes after one rak'ah and stands beside him, he must repeat one rak'ah, and that one is invalid.

Answer : His prayer is not invalidated, because the rak'ah, when he prayed it without an excuse, was invalid in the first place, and if the rak'ah is invalid, then what is built upon it is invalid.