Nancy and Macey, unable to contact Fred, worried that Mya might have been sidelined due to Ava's influence. They recognized Fred's pattern of fleeting commitments, except to Ava, and feared Mya's innocence and naivety made her less favorable in his eyes.
They speculated that sending Mya away for treatment was a strategy to distance her, which would ultimately lead to him abandoning her. Comparing Mya to Ava was no contest, as even they had not seen a woman more attractive than Ava.
If it were