Shabbos 4 – שבת ד

The Gemara tells us that a person may not transgress in order to prevent his friend from sinning.  This implies that one may do so for his own benefit.  Since we know that ‘kol yisrael areivin ze lazeh’ and that all Jews are responsible for one another in the same way as they are responsible for themselves, why then would he not be permitted to do for a friend exactly as he would for himself?