The poor thing is going thru such a trauma as her best friend just died and she could not do anything to prevent the death. On top of it she is blamed for Tanya's death.
Now Karan sould not leave her alone for a second and sould make her comfy and bring her back to SN. I am sure Tanya will rise from dead and everyting will be just the way (i.e miserable) it is. Just pooor Nandini need not suffer and she dun need to grieve. God bless her