Memorising exceptions for soluble salts
Understanding why it happens unfortunately will not help her to remember. This is the classic case where you have to find a dirty way to remember them. Maybe you can make some practical experiment I remember most of them with a practical application of the salts.  

There are a lot of online resource regarding Mnemonic Tricks to remember solubility rules, these are two video on youtubes.

Memonic tricks

Song!

Quoting the song:


  This is solubility it's so crazy, don't be a fool and follow these rules!