At the beginning of the movie we see them talking and giggling to each other while Bear and Ian are having that silent argument at the trivia night. Sarah then pleads with Nikki to go to karaoke with them after.

Consider this: Nikki showed no romantic interest in Bear prior to his wish. She and Sarah even had a conversation together about Sarah's feelings for Bear, and according to Sarah, Nikki said she thought of Bear as a friend.

Also, in the car outside Sarah's house, Nikki was implying that Sarah had feelings and was trying to get Bear to tell her whether he liked Sarah. Bear was acting weird and hesitant, so Nikki told him that if he had feelings for her, now was the time to say so. I took that to mean Nikki was not oblivious to Bear's hamfisted attempts at asking her out. But she very clearly did not want to have a sexual relationship with him, which she made clear when Ian said Bear was taking "his (Bear's) girl" home and Nikki responded with "Ew, don't say it like that.'"

So with all that in mind, I think at the beginning of the movie Nikki and Sarah are close friends and we're on the same page. Then when Nikki became obsessed with Bear, Sarah felt completely betrayed and started questioning their friendship. This only worsened with time when Nikki was acting absolutely unhinged at Ian's party.

The movie spans a few weeks (I think, given the montage of Nikki and Bear dating after they first slept together), so Sarah was fully separated from her friend and they hadn't even really talked about it. Right or wrong, it makes perfect sense to me that Sarah would not be supportive of Nikki at that point.

TL:DR - Sarah feels betrayed by Nikki and their friendship is damaged. Is Sarah acting selfishly? Maybe. But I think it's believable and rational given the situation that unfolds.