If circumstances allowed she would have take Rohan out to lighten his mood. But again, she was getting married and its was just not appropriate for her to be roaming around with him. For the last few days, normal routine in the Verma house has revolved around her wedding and hence Nidhi tried her best to get Rohan involved and be as normal as he can!
If Rohan left the night/ morning after she rejected him, Nidhi would have let him go. But he was trying to leave after her wedding was fixed. Now that would have put them all in an awkward situation. Rohan's absence at such an important occasion would have raised some eyebrows and Ashu-ARmaan would have raised this issue. Was this the right time to confess the truth to Ashu? I think not! Maybe one day after they are confortable with each other Nidhi will reveal the truth and Ashu will understand, but not now. She does not want Rohan to loose Ashu's respect for him or his trust.
Even though Rohan loves her now, her friendship with him is pure. She has no doubts in her mind about it. She has loved only Ashu and Rohan is always a friend, very clear on both counts.