I would say the problem is not the "wenn"; I think that "etwas toll finden" changes its meaning here. The back of my head seems to say that "es toll finden, wenn..." means something like "to enjoy something". As opposed to "es toll finden, dass...". Here, it has its literal meaning. ("to find it great that...") "Sie findet es toll, wenn sie jeden Tag ihre private Modenschau machen kann." and "Sie findet es toll, dass sie jeden Tag ihre private Modenschau machen kann." have two different meanings. With "dass", it says that she's enjoying the fact that she's in the state to do this every day. This sentence has NO feeling of the passing of time, that means, it does not ention that her feelings about this change depending on time of day or activity. The "wenn" version means rather "She's delighted when (whenever) she can make her private fashion show very day" Comparable to "Die Katze findet es toll, wenn sie gestreichelt wird." "The cat enjoys it when it is petted/stroked." We know that she is enjoying the moment when it happens. We know nothing about the time in between those moments.

I agree that "wenn" is not the best word to use in that sentence.. but even a lot of native German speakers tend to easily make mistakes when it comes to the usage of words like "wenn". I think what they meant to say is: "She constantly buys new things and thinks it's great, 'that' she can have her private fashion show every day. In that case, the word that should be used instead of "wenn" is "dass". So it should be: Ständig kauft sie neue Sachen und findet es toll, dass sie jeden Tag ihre private Modenschau machen kann.