You really need to clarify what happened.
Did the teacher tell you he did this to the little girl? If not, that's quite strange as it's very serious even at 5. I'd also question the fact that he was unsupervised - was that just his take on it? If I've kept children in at play, I've always been just outside the room marking or tidying. That may make a child think they're alone but obviously I'm right there.
As for the picture, I'm afraid I wouldn't have let him colour. If a child in my class had done something serious, they'd have sat in silence. It's not often that I've kept a child in but what's the point in keeping them in and letting them do something enjoyable?
Are you sure she threw the picture away? It may seem cruel, but it was a punishment after all. Maybe she put it somewhere for him to have later. Again, needs clarification.
It also should have been clarified at the time, because if that had been my son, I'd have wanted to have a stern word too, had he done something like that. I'd have wanted to ensure it was never done again, yet it sounds like maybe you've pandered to him.
Next time, if there is one, try and speak to the teacher immediately. This time, I'd say move on.