Your air carrier need not even know of her Italian passport. Its concern is that she won't be denied entry to Brazil and that is accomplished with her Brazilian passport.
If by some bizarre chance, you are asked whether she is a dual national, you would, of course, give a truthful answer. But, there is absolutely no reason to volunteer information not directly asked for. That applies to both check-in (as well as boarding) and passport control. The more complex you make it, the more complex it becomes.