How to override an unpluggable parent theme function?

Since the function is used in your child theme’s template there’s nothing stopping you from using a function with a different name. So you don’t actually need to plug the function, you just need to create a function that does what you want and use that instead.

So copy ale_filter() to your child theme, rename it something like ale_child_filter(), change it however you like, and then just change your template to use that function.

You only need to plug a function if the function is used somewhere that you can’t overwrite with your child theme. Plugging it would allow you redefine a function when you don’t have access to change where the function is used. Since you do have the ability to change which function is used, plugging the function is unnecessary.