Rashami will take time to come out of it and think rationally. The ones who have ever face cheating in a relationship must have known that how the cheated one tends to forget the cheater and give him a chance more because the heart is not ready to believe all that. It's human nature. Nothing can happen with the speed of light. She will take time to get detached from Arhaan.
Unfortunately, she will be judged for being cordial with Arhaan after this. But I just wanna saying nothing happens instantly. A hell lot of things must be going on in her mind & hear right now. She needs time & personal space to think rationally and make the rightest decision which is not possible in BB house.