2,655. Fulfilling Another Person's Oath

Hilchos Shevuos 5:2

The reason that one who takes an oath regarding another person’s future actions isn't liable to the penalty of lashes for taking an oath in vain is because the other people might listen to him and fulfill his oath. Therefore, when he’s warned when taking the oath, the warning is in doubt. In such cases, one isn’t lashed unless the prohibition is explicit in the Torah; this will be discussed IY”H in Hilchos Sanhedrin. People are not obligated to fulfill the vows of the others unless they responded amen, as has been addressed.

Hilchos Shevuos 5:3

If the other person fulfills the oath, he is praised for not enabling oaths to be made in vain.