1
This coat is her.
This coat is hers.
Her is a possessive adjective, also called a possessive determiner. We use it before a noun: ‘This is her coat.’
Hers is a possessive pronoun. We use it without a noun: ‘This coat is hers.’
2
This bag is her’s.
This bag is hers.
Do not use an apostrophe in the possessive pronoun hers.
See MINE