Calves will dash across the road to be with mama. I always watch out for that. If it was daytime, no excuse. If night and the cow was black, there's a chance she didn't see it in time to stop.
If the area is posted as open range, she bought a cow.
The only other time I know of someone who hit a cow, it was already injured and laying down on the road. Black cow, black road, dark night (no moon). He and his 4WD went right over the cow.
I can understand night time, black cow. But I have a SIL that goes blasting down the road, where there are pronghorns, deer, buffalo, mules, big horns, she'll just yack away.
I avoid riding with her if possible. If I can't avoid it. I am the hyper alert spotter!