If div in iframe has child with specific class do

I want to add a class to the parent if the child has a specific class. The problem: It's in an iFrame and I'm not very good with jQuery.
It don't really has to be jQuery, any other way would be also great. Just notice: The iFrame is on my domain, but I can't access it, because it's generated by a plugin.

If you have any ideas how to fix it, I would appreciate it

My HTML looks somewhat like this in devtools:

<iframe src="#" id="iFrameResizer0">
<div class="book-day">
    <button class="disabled">Button Text</button>
</div>
<div class="book-day">
    <button class="active">Button Text</button>
</div>

</iframe>

and my jQuery:

$(document).ready(function () {

$("#iFrameResizer0").contents().find(".book-day button")
    if ($('.book-day button').hasClass('disabled')) {
     $(".book-day button").parent().addClass('disabled');
}

});

if everything works correct I want my html looks like this afterwards:

<iframe src="#" id="iFrameResizer0">
    <div class="book-day disabled">
        <button class="disabled">Button Text</button>
    </div>
    <div class="book-day">
        <button class="active">Button Text</button>
    </div>
    
</iframe>

Devtools: