Assalaamu `alaykum
I don't know about the curse of Allaah upon this person, but a person who sits with a group of people whilst they are sinning (and he is sitting with them although he is able to leave them) and remains silent and does not rebuke them or speak out against them- has done the same sin as the people sinning around him.
Allaah, commands us to not sit with people who are ridiculing and denying the ayaat of Allaah- untill they speak about something else. Allaah said:
فَلا تَقْعُدُوا مَعَهُمْ حَتَّىٰ يَخُوضُوا فِي حَدِيثٍ غَيْرِهِ ۚ إِنَّكُمْ إِذًا مِّثْلُهُمْ
So, do not sit with them until they enter into another conversation (but if you stayed with them) certainly in that case you would be like them. (Soorah al-Nisaa': 40)
This is why Imaam al-Tabari in the explanation of this ayah said that if you are sitting with a group of people who mock the ayaat of Allaah and deny them- and you do not leave them... you are just like them (i.e. you are like them in this action of their's). This is because you have sinned by sitting with them in this gathering and you listened to them mocking the religion of Allaah and denying it and thus fell into the same sin as them- because you did not leave them, nor rebuke them.
That is why we see that when one of the munaafiqoon was mocking the religion of Allaah at the time of the Prophet :saw2:, and the people in his gathering did not rebuke him nor abandon him, Allaah revealed this ayah about 'all' of them:
لا تَعْتَذِرُوا قَدْ كَفَرْتُم بَعْدَ إِيمَانِكُمْ
"Make no excuses; all of you have disbelieved after you all believed."
Allaah mentions the disbelief of ALL of them and not just the one person who mocked the religion of Allaah.
This is the danger of sitting with a group of people when they are sinning.
A similar principle was mentioned by Shaykh Ibn `Uthaymeen rahimahullaah.
Wassalaamu `alaykum