Even where many other persons are present, an offence (such as under Section 509 IPC) can be committed. However, it is possible that the complaint may be false. Now that the FIR is registered, the police will investigate the case, and if sufficient evidence is found, the police will charge sheet the case. So, your friend (i.e., accused) should present his side of the story with evidence (during investigation, and if needed during trial also) to show that no such offence was committed and/or that the complaint is false. Luckily for your friend, the offence under Section 509 IPC is bailable, so in case he is arrested he would immediately get the bail.