Is it rude to enjoy schaudenfreude? The answer is no; it's just rude to gloat about it.
I think we actually had a thread about that very question some time ago. I think the general consensus was that, as with many things, if you manage to keep them entirely in your head, and they don't leak out into your behavior, it's not rude. I think this applies to Madeleine in an interesting way--etiquette is not
necessarily concerned with why
she went to the play. It was open to the public and she paid her money (or whatever) like everyone else, and while there didn't behave in any objectively rude ways. So she's clear on that point. Telling the OP/others about what happened afterwards may or may not be rude--it can be hard to pin down when FYI turns into bad gossip.
But sometimes I think it behooves us to examine our motivations ourselves, what they say about us, and how we feel about that, kind of like with schadenfreude. And also, in Madeleine's case, how her appearance at the play might have been perceived
by Carol--not that other people's perception should be the sole reason we do things, but I think EHell has been very good for me, at least, by helping me to see others' POV in situations.
Honestly, the thing I personally would like best in this situation, is if somehow, through counseling or whatever, Carol suddenly realized how much her behavior was harming herself and her son, saw the light, and became determined to turn her life around. And then was able to do it. From what's been described here, her life seems so desperate and sad to me, and I feel bad that that's spilling over onto her child. I don't think she should have no consequences for her actions, though. In fact that may be the only thing that wakes her up to reality.