The battery is necessary to start the bike. Once started, the stator and rectifier/regulator will provide power to keep the bike running (assuming they both work properly).
Bring jumpers and jump to a known good car battery (car engine running or not is of no consequence), or, bring a battery jump box along with you.
When I go to look at a bike, I always bring a jump box, some oil and a can of starting fluid - to hedge against a bad battery, low oil level and plugged carbs or old gas (very common with bikes that have sat for a long time). This way, I almost never have to depend on the seller's statement that 'it runs great, but the battery is dead or the carbs need cleaning or the gas has been in it for too long'.
They may push start it for you and it runs great, but if you can't get it to crank over with a battery box/jumpers, you also won't know if the starter one-way is shot. Ask me how I know about this trick, dangit!

Yes, that was the first thing I had to do when I got my Rappy home - it push started and ran fine with the dead battery, so I bought it... after getting it home and charging the battery, boy was I dissapointed!
$250 and some hours of my labor later, to replace the one-way, it finally started with the start button. That's also when I found this forum, thank God! Wouldn't have handled it without some help.
These days, if I can't make it at least run on starting fluid and a battery box, I'm not going to consider buying it.