What is the life story of Mandodari after Ravana's death?
Probably she married Vibhishan:
After the death of Ravana, Rama advises Vibhishana to take Mandodari as his wife, even though he already has a wife. A theory suggests that Ravana's race may have had matrilineal families and thus, to restore order in the kingdom after Ravana's death, it was necessary for Vibhishana to marry the reigning queen to get the right to rule. Another theory suggests it may be a non-Aryan custom to marry the reigning queen. The marriage between Mandodari and Vibhishana is purely an "act of statesmanship", rather than a marriage based on their "mutual sexual interference". Mandodari may have agreed to marry Vibhishana, her younger brother-in-law, as this would lead the kingdom to prosperity and stability as allies of Rama's Ayodhya, and she would continue to have a say in governance. Another reason for the marriage is as an alternative to suicide for the widowed Mandodari, which is averted by Rama.
Some legends state that in order to prevent Mandodari from entering the funeral pyre after the death of Ravana, Rama requested Vibheeshana to marry her in order to enable him to rule Lanka wisely. Refer