My hair is longer than her's (is) some speakers and good fluent natural English native born speakers do indeed add on an extra "is" at the end of such a sentence to clarify the situation. Surely my hair is longer than hers =(plural females more than one usually taken as meaning on) is incorrect. Please check your answers and if required edit and correct the question.

Hi Jahi I see your points which I accept. The problem may in part be due to Italki teaching American English yet still allowing other Englishes onto the site causing some confusion. Also ı think "longer than yours" may also be an acceptable answer.