Could be many of things! First, I would start with gas.

I know my previous bike was kind of picky with what gas I put in to it. Did he just take it out of storage? If so, maybe he didn't put fuel stabilizer in?

Did this just happen out of the blue? If so, what was the last kind of maintenance he did? Did he perhaps forget to plug in the MAF sensor?

This is all speculation until you give us some more info because really, it could be something really stupid that he overlooked.