Is it turning over? (make sure the piston moves, it could have frozen in place if sitting awhile) Have you checked the safety switches? (kickstand, clutch, on/off) to make sure they were not engaged? Clutch has to be held in to start,, even in neutral...its a Suzuki thing (not sure you would know this)
What is the voltage going to the starter?(you might have spark but not enough to keep it running)
If you have not gone over all the electrical connections (might be your friend is not as knowledgeable as you are about bikes) this bike tends to vibrate a bit.
Hope my little contribution helps.... its all I could think of right off the bat.
Um, fuses?.... don't think it applies, but you never know! finicky critters sometimes.... ya gotta learn her ways to satify her!