Hey mate I also have a 99 hayabusa that has a similar problem. The bike cut out as I was walking it up a ramp. The oil pressure warning light came on and stayed on even when the the bike was switched off and the key removed. The bike wont start and the instrument panel will not light up. I had to remove the battery to make sure the light didnt drain it.
I called my mwchanic who had recentky replaced my fuel pump and spark plugs and he had no idea on wgat could be the cause.
As it sounds similar to the issue you had with your friends bike, could you give me a little bit of additional information on the cause and how was fixed and if it was an expensive exercise. That way I can give the mechanic an idea of what the problem might be and how it could be fixed.