The Gemara has a long discussion on how we learn out that עדים are מחייב a שבועה. At one point the Gemara says that עד א יוכיח שישנו בהכחשה. Tosfaos there is גורס the הזמה as well. Tosafos explains that the fact that he doesn't pay money is just because he is not מחייב money. The (קצות (סימן ל"ח ס"ק ה 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), brings an expicit Rambam (הל' עדות פרק כ"א הל' ה) 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 ..."
Based on the Rambam the Michas Chinuch is surprised why the קצות said he is not sure that there is הזמה by an עד אחד.
However if you look in the Frankel Rambam you see that there are different נוסחאות in the Rambam whether it should say עד א or not and therefore the proof from the Rambam is not so clear cut. Also see the חזו"א there.