1990 88 totally dies no power to nothing

My mother in law's car totally died going down the road one night, no lights, engine nothing after they got it off the road it cranked back up and they got it home. About three weeks later she went to the store and it would not do nothing as if the battery was dead so a guy went to give her a jump and only hooked up for a min it fired up she went two blocks to auto zone they checked the battery and alternator all checked fine, also had the battery checked two more times and still at 100%. any help would be greatly appreciated thanks

Look for corroded or bad battery cables and grounds especially right at that double cable on the battery.

Thanks I will check.