The (קצות (סימן ל"ח ס"ק ה points out that we see from Tosafos (ד"ה עד אחד יוכיח) that there is הזמה by an עד אחד as Tosofaso says that the girsa of ובהזמה is correct. Tosafos explains that the fact that he doesn't pay money is just because he is not מחייב money. The קצות says that the נפקא מינא would be in a case where he is מחייב money such as where he is מחייב a שבועה that the person can't take and the person then has to pay. In that case if he is מוזם the עד would need to pay. The קצות ends off that he is not sure that there is הזמה by an עד אחד.
The Minchas Chinuch (מצוה ל"ז), points out that the Rambam (הל' עדות פרק כ"א הל' ה) says explicitly that there is הזמה by an עד אחד by a Sota:
"בא עד אחד והעיד שזינת אחר הקינוי והסתירה, ונמצא אותו העד זומם--משלם כתובתה ..."
And therefore the Michas Chinuch is surprised why the קצות said he is not sure that there is הזמה by an עד אחד.
The fact is that there are different נוסחאות in the Rambam whether it should say עד א or not and therefore the proof from the Rambam can be disputed (עיין חזו"א שם).