I am sorry to hear this about your friend. Unfortunately it is hard to say why he is in a coma. It could be from a stroke, could be from a loss of cerebral blood perfusion due to the surgery. If the MD's havent determined why, then it is hard to say...Good luck and keep your head up. Even in a coma people can hear others talking to them. Visit him if possible and play music or read to him. If there is not significant neurological damage, it is only a matter of time before he wakes up...
This type of scenario is usually due to an intraoperative stroke. Of course being in the post-op state, having been intubated, and having received various mind-clouding drug agents have a reversible effect on the brain. Until all that clears out, his doctor will assess his neuro status in conjunction with neuroimaging (MRI). So technically speaking this is not a comma, instead, it is delirium and will likely improve and at this point it is difficulty to say what type of recovery to expect.