Can you give more details? Why does your friend think this?
There are a number of mental health experts committing on this. One example is:
http://gto7.wordpress.com/2008/09/23/understanding-obama-the-making-of-a-fuehrer/
The mere fact that he has never released his medical records...as almost all of the presidential candidates have done over the past twenty-five years....and the media never questioned that...would make most folks laugh. The question here...if you discovered he was on drugs....for serious mental health reasons....would the public demand he resign? My guess is that 50 percent could not bring themselves to demand that.