Take a digital volt meter and a 10mm socket wrench with a 3" extention and ratchet handle with you to take the seat off and check the battery and charging system.
Unlock the toolbox on the seatback to get to two 10mm bolts holding the seat on.
Hopefully the new battery is a Maintenance Free-Absorbed Glass Mat (MF-AGM) style with a "sealed" top, (looks like this)
not an old style wet cell (like this one), with 6 removable caps to add distilled water regularly.
Battery voltage at the terminals should be 12.5-13.2v with the engine off.
Watch the voltage as owner starts the engine and slowly revs it up to 5,000 rpm.
Voltage should gradually climb starting at ~2500 rpm to about 14.5v at 5k rpm.
If it does, the stator/alternator and regulator/rectifier are probably in good shape.
Ask if the rear splines between the diveshaft and final drive were lubricated when the rear tire was replaced the last time. Unless he is a member of this forum, he probably won`t know. Many/most dealership shop "mechanics" don`t even know which splines to lube, how to do it or which lube to use, (Honda Moly60). Don`t worry though, we can teach you to do it yourself. Even if you have no wrenching experience. Honest, we really can!
Check the tires for wear and cupping on the tread along with any checking/cracking on the sidewalls.
Also check the manufacturing date code on the sidewall.
Even if the tires look brand new, if the code indicates tires are 5 years old or more, consider they should probably be replaced at a cost of $2-300. Others may consider they should be changed before they get even that old.
Open the tank cap and take a look inside with a flashlight. Look to see if it is clean or rusty. Sniff it to determine if the gas is fresh or old and rancid. You said the bike idled well, so the tank and gas are probably fine.
The owners asking price sounds fairly reasonable to me at the beginning of the riding season. See what others closer to you think is a fair price. Where do you live?