Shutting off. Battery charge drops suddenly.

OK this is my first post and its not my car (its my father in laws). The other day his 05 prius (170k miles) shut off while he was driving. He pushed it home and I sent him a link on how to check the 12v battery. He said it showed it was low so he used his golf cart battery to go and take it for a test drive to see if that fixed the problem. In the drive way at idle his hybrid battery charge went up half way but when he went to drive it he got 1/8th of a mile down the road and it dropped to 0 then the car shut off again. He said the display says something about the battery (I havent been over to look at the car). They are sure its the hybrid battery, but if it was me I'd go to the dealer and find out for sure. My husbands aunt had her hybrid battery go out and hers did other things, like go into turtle mode and a/c blew hot. I think her inverter pump went out and she kept driving it till the battery got messed up, but anyway...What does my father in laws symptoms sound like? Is there any extra at home testing that can be done? Thanks for bearing with me, I dont know all the lingo so talk slow and use small words.