"NO"
Is a complete sentence. I wonder how future FIL will react should you use that sentence on anything he is doing.
Personally, that is an insult in my book. A back handed way of saying he thinks you two (and mind you he is not a psych dr. or a therapist) need to work on your relationship, when in reality his son needs to work on his recovery. And then not even to ask first, but just go ahead and arrange it and PAY FOR IT. What an insult!!!!!
Okay.
I'm going to wish you a happy life and just say that when it starts to go to he!! in a hand basket we will still be here.
Love and hugs,