The same thing happened to me with my quadruple bypass surgery last October. I had high frequency hearing loss before the surgery, but shortly afterwards realized that the hearing in my left ear had diminished substantially. I have seen an ENT and had an MRI of my head and they have not found any physical reason for the loss. The best explanation so far is that the heart/lung machine used during the surgery did not supply sufficient blood to my ear, causing the loss. All I know is that I so far do not have any improvement in the situation and no doctor has offered any solutions.
Don't know... what did the doctors say? Possibly a stroke (blood clot) in the inner ear? I'm just guessing.
I'm also guessing that the prognosis for return of hearing isn't good (it usually isn't with a profound sensorineural loss). Have they referred him to a neuro-otologist (inner-ear specialist)?
Good luck to him. If his hearing doesn't come back, there are options for routing sound from that side to the good ear--an audiologist can advise him.