Hi,

First of all Brasil and regarding your question Sao Paulo are not that dangerous as people think they are. It is true there is a lot of poverty and it leads to crimes in the city which is considered as a the economical center of Brazil. You have to know where you are going and don't show off any of your wealth try to mix in with the locals and you'll be fine. 

Brazilians have nothing against tourists with local girls , believe me it's OK.  The Local girls usually hook up with the tourists because they can supply them more then they can get by them self. Anyways I am sure your friend knows where to go and there is a lot to do in Sao Paulo a lot of people say there isn't anything to do because the city is so big and they can't find anything. Having a support of a local will help you a lot.

 Just in case the areas you should not wonder around at night are near the centro.

You will have a blast..

Enjoy