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Hello,

In the sentence, "It had been Thomas's and her's secret hiding place when they were children." Is the apostrophe after 'her' correct? It seems that it should be, but my MS Word Grammar check does not recognize it. Thank you for your great service.
Comments 
Hello Anon, you shouldn't use "s" after her. If you say her, this will already mean that "she" possess something. Check possessive pronouns please. Emotion: smile
Thomas's and her secret (they share the secret)
or
Thomas's secret and hers (each has a secret)
Note where the apostrophes occur, and where they don't. The rows show the subject form, the object form, the possessive adjective form, and the possessive pronoun form, respectively.
I you he she we they it Kate the child the boys

me you him her us them it Kate the child the boys
my your his her our their its Kate's the child's the boys'
mine yours his hers ours theirs its Kate's the child's the boys'
Note that there is no her's.
CJ
AnonymousIt had been Thomas's and her's secret hiding place when they were children
It had been Thomas and her secret hiding place when they were children.

Since they hid in the same place, an apostrophe after Thomas is not necessary.