shifter

Had a look at a mates sv yesterday. He's had the battery go flat a couple of times when the bikes been left standing a while.
Needless to say I suspect a reg/rec problem. I've checked the alternator off load and its output is fine. but I'm seeing 17v at the battery terminals at 5000rpm, I belive it should be more like 15v?
So my question is, if the output is too high, can this cause the battery to go flat?
I allso checked for earth leakage (no problem there) and the battery was fully charged on an optimate which reported no problems either.
The bike doesn't have an alarm fitted by the way.
I think if it was my bike I'd change the rectifier and take it from there, but I don't want to waste someone elses money if it could be something else?
Second opinions would be much apreciated.