It's possible the woman didn't know someone had been convicted and imprisoned in the girl's death, or even that the girl had dies. The article doesn't explain how she came to know this. For all we know, her father was driving the motor home, did the hit-and-run without anybody else knowing (especially since it was apparently just the mirror that hit the girl), and confessed it to his daughter on his deathbed. If she has known about it all along, she will be facing some liability, though perhaps not a huge amount, assuming she didn't actually testify to the contrary. After all, if she didn't testify, she is hardly the primary cause of a man having been convicted for murdering a girl with a whiskey bottle, when in fact the girl was hit by a motor home.
If I remember correctly, someone (dad? husband? some relation) told her on a deathbed confession that they were driving the Winnebago and the passenger mirror hit the person on the side of the road.