Why can this not be " He does it for you "?
because the 'sie' is not capitalized.
Can it mean 'for her'?
"Es tut für sie". "Do it for her". As in that Simpson's quote.
The command form would be Tu es!, though.
why not 'er tut es fur ihr' rather than 'sie'?
für is always followed by the accusative case. For sie, that is sie. To be ihr, it would need to be dative.