Imam As-Sadiq (a.s.) said:
لا يَنْبَغِى لِلْمُؤْمِنِ أَنْ يَجْلِسَ مَجْلِساً يُعْصَى اللّهُ فِيْهِ وَ لا يَقْدِرُ عَلَى تَغْيِيرِهِ
It is unjust for believers to participate in a feast where sin is performed and they do nothing to change the situation.1
Even if one does not commit the sin and does not cooperate with participants of the feast, participation in a sinful feast is itself a sin . It amounts to endorsing a sin unless one participates in order to remedy the situation and perform the critical duty of enjoining good and forbidding evil.
Furthermore, when a person keeps observing sinful scenes with indifference and without protest, it darkens his spirit and decreases his sensitivity so that sins no longer appear so bad to him and this can lead him to commit sins.