I saw this yesterday in a real-estate advertisement.

Problem:
“HER’S” is a nonsense word.

Explanation:
The pronoun “her” is:

• the objective case of the pronoun “she” (e.g., “Give this green handbag to her.”);
• the possessive case of the pronoun “she” used as an attributive adjective (e.g., “Her handbag is the green one.”);
• the dative case of the pronoun “she” (e.g., “I gave her the green handbag.”).

The pronoun “hers” is a a form of the possessive case of the pronoun “she” used as a predicate adjective (e.g., “The green handbag is hers.”).

In contrast, “her’s” is a nonsense word.

The woman who wrote the real-estate ad meant to say that the walk-in closet was suitable for a man and a woman simultaneously, and she could have used “her” or “hers”:

• Using “HIS-&-HER” would have meant that she wanted to say that the walk-in closet would be equally possessed by the man and woman.
• Using “HIS-&-HERS” would have meant that she wanted to say that the walk-in closet had a “his” area and a “hers” area.

For fun, I searched Google for each of the following words (with the quotation marks to avoid modified forms) and got about the indicated numbers of matches:

• “hers” — 23,800,000 matches
• “her’s” — 1,290,000 matches

Although some of the “hers” matches related to acronyms (e.g., for Higher Education Resource Services), this still tells me that Web authors have written the word correctly vs. incorrectly by a ratio of some 18.4:1, which is okay, but the fact that the Web contains more than one million instances of “her’s” is disappointing.

Solutions:
“HIS-&-HER WALK-IN CLOSET”
or
“HIS-&-HERS WALK-IN CLOSET”