From Teflpedia

She’s is the English contraction of she is or she has.

The contraction consists of a pronoun (she) and verb (is or has).

English language learners may make the language error of using she’s instead of the dependent possessive pronoun her, e.g. *She’s name is Sarah.

This comes from the overgeneralisation of the rule “add the morpheme +s to form a possessive”. This seems to be acquired quite late in many cases, despite the correct her name is Sarah being taught at a very early stage.

A similar error is often made with its.