Toyota Prius No Start

I received a call this morning from a friend with a 2016 Prius. She had driven the 25 miles to town and after having been in town a couple of hours the Prius wouldn’t start. She called me and asked if I would jump the battery.
Now my knowledge of the Prius is limited. I know that the Prius has a smaller 12 volt battery and a large high voltage battery. I jumped the smaller 12 volt battery with no results. After having jumped the smaller battery, I measured the voltage of the battery and it was 8.6 volts.
My reasoning at that point was that the smaller battery might be defective. However, the Prius did start this morning after sitting overnight. She wanted to go buy another battery. I reasoned that the 25 mile trip should have charged the battery and the problem might be the charging system. I was worried that if we replaced the smaller battery, it might not solve the problem. She had an important event back at her home, so I convinced her to have her Prius towed to our local Toyota dealer and take my van to get her back home.
I admit that I don’t know much about the Prius. I didn’t want to replace the 12 volt battery if the problem was the charging system. Was I on the right track with my thinking?

Absolutely , I have read that even jumping a Prius is not like non Hybrid vehicles . I think you dodged a bullet . Having the dealer solve this is the wisest move.

Thank you for your vote of confidence in what I did. In the Toyota Prius cars that I have ridden as a passenger, the electric motor seems to get the car moving and then the gasoline motor kicks in. I don’t know whether or not there is a starter motor as on a conventional internal combustion engine non hybrid car. I get upset about technology that I don’t understand, particularly since the Prius has been around since the early 1990s.

Not in the conventional sense. There are two “Motor-Generators,” designated MG-1 and MG-2. MG-2 is constantly connected to the engine and also acts as the starter. No solenoid, flywheel, etc. You can read more here: https://priuschat.com/threads/is-the-prius-starter-different-from-regular-starters.18227/

@davepsinbox_157004 Thanks for the link to the information. From what I read, I am not sure that attempting to jump start the Prius by hooking the jumper cables would have done any good. I also have no idea how the Prius recharges the smaller 12 volt battery.
I have driven the Honda Civic Hybrids and the Ford Escape Hybrid, but I have no knowledge about how the gasoline engine was started.

It’s a little more complicated than with a conventional vehicle but it can be done: https://www.downeasttoyota.com/blog/how-to-jump-start-a-hybrid-toyota-car-or-crossover/

Update: the problem on my friend’s Prius turned out to be the 12 volt battery. I guess it had enough power before she made the 25 mile trip.into town to allow the Prius to power up. However, after sitting for three hours, it went dead and wouldn’t take a jump. I had the same thing happen some years back on a non hybrid vehicle, so it’s possible for a battery to lose power unexpectedly.
I did read up on how hybrid vehicles operate. Toyota is coming out with a revised Sienna for 2021 and will only be available as a hybrid. Much as I am fascinated by new technology, I think I will stick with my present 2017 non hybrid Sienna.