Disable a button in the Details table based on a field in the Master table

In phprunner 10.91 I have a loan system with a table called inventory (with id called inventory_id) and another table called partners (with id called partner_id). The table that relates them is called loans and has the fields inventory_id and partner_id. It also has two fields to control the date. One is the date_pres field that stores the date on which the loan is made and another field called date_dev that stores the date on which the loan is returned.
What I need is that if the date_dev field is empty, that item cannot be loaned.
The loans table works correctly when adding a loan the inventory item that has the field loaned = 1 does not appear. The problem is when I use spreadsheet mode within the inventory item. Then, the Add new button is always available even if the item is loaned and does not have a return date.
In a BeforeAdd event I can add a code like this:

But it would be much better if we could disable the add button when the master table "inventory" had a 1 in the "borrowed" field. Or at least use a sweet alert

I try disable inlineadd button in ListPage->Before Display`global $conn;

// Consulta para verificar si el artículo ya está prestado
$strSQLExists1 = "SELECT * FROM inventario WHERE id_inventario = '".$values["inventario_id"]."'";
$rsExists1 = db_query($strSQLExists1, $conn);
$data = db_fetch_array($rsExists1);

// Verifica si el registro existe en la tabla inventario
if ($data) {

$pageObject->hideItem("inline_add");
}

}
`
But .$values["inventario_id"] seems to be empty

Any idea? Thanks