Q. When one notices that the chazzan began chazaras hashatz even though there are not ten people in the room (a minyan of exactly ten, and someone went to the bathroom), is one obligated to tell him that he should stop? Or should one continue reciting the Amidah and allow the chazan to make nineteen blessings in vain?
A. Shulchan Aruch (O.H. 55: 3) rules that once the repetition of the amida has began with a minyan, it is continued even if one left and there is no more ten present.
However, Horav Shlomo Miller’s Shlit’a opinion is that if there weren’t ten people present at the beginning of the amida, the chazan should stop and begin again when the tenth one returns to shul.
One still in the amida, may also advise, preferably by signs, to stop and wait for the tenth to return.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a