Not that it negates your DD actions, but I slapped DH crazy hard frantically trying to kill a daddy long legs. On his naked arse as he'd just got out the shower. So hard that a perfect, red, raised imprint of my hand appeared in seconds. And was gone without any trace about 5 mins later.
So, I'd expect the mother to be cross, but the red marks hours later, when the school didn't feel it worthy of notifying either of you, I think sounds suss. If there were red marks hours later, just think of the mark at the time. That wouldn't have gone unmentioned imho.
Clarify exactly what happened from the school.
Then address your DD about how this is completely unacceptable, in the way you know she will understand best.
DS punched someone before (he has ADHD so quick to anger) and I know the feeling, your DC is "the one that slapped Jane" and it's awful. All she can do is apologise, and what she's doing with the card sounds a good idea.
But then, equally, move on, and don't engage any more with the mother.