Uttering falsehood is one of the major sins in Islam. Islam is a religion based on truth and therefore does not accept spreading of falsehood. In the Holy Qur'an believers are invited to "advice each other to the truth" (103:3). In the said case, it is incumbent for a Muslim to act upon the principle of enjoying good and forbidding evil. Its obligation is based upon the following conditions: the person enjoying good and forbidding evil should be able to distinguish between good and evil, there should be a possibility from stopping the transgressor, we should be sure about the persistence in sin by the person whose behavior should be rectified (meaning that he does not repeat the sin), and we should be safe from harms that may bring greater damages while performing this duty. If all these conditions are met, it will be necessary to enjoy good and forbid evil by advising the person uttering falsehood to rectify his behavior. If he does not listen to the advices it will be allowed to use stronger words, and if this does not work too, it will be permissible to take practical steps according to wisdom and circumstances. If all of these are not possible, and other people too are unable to fulfill this kifa'i responsibility, to reject falsehood within your heart will suffice insh'Allah.
With prayers for your success.