That's why I did the second test with 8 WIS and 20 CHA - the game doesn't do that either. The only possibility I can think of is if he had something else (a class or a race) that used CHA as a spellcasting modifier. This would be weird, but sometimes the game does some weird things.
My first test was Druid 6/Wizard 1 with INT 10, WIS 14, CHA 18. Owlbear's Rupture had a DC of 11. That's right - it's used INT instead of WIS, even though WIS was higher. I have no good explanation for this.
My second test added in a level of Warlock. Now Owlbear's Rupture DC is 16 using CHA.
My third test was Druid 6/Bard 1 with INT 8, WIS 14, CHA 18. This one used CHA right out of the gate for the DC.
My fourth test added a level of Wizard. Back to low Rupture DC using INT.
My conclusion: multi-classing can do some really weird things to wildshape DCs. The order of the multi-class matters, which is bad. A multi-class can lower your wildshape ability DCs, which is bad. If I had to guess, the wildshape is using the last class tab in your spellbook view to determine the DC it's going to use. I don't know if you can have a racial tab in that view, but if so and if it has a spellcasting DC on it, your wildshape might use that one. Just a lot of really weird behavior.
