1. "Sadly, I cannot get this man to accept the notion that the Bible really is the word of God."
Ok, let's start there. The Bible never calls itself the Word of God. That's part of the problem right there. The Bible speaks of the Word of God quite often, but it never has the self-awareness to declare itself to be that Word. Maybe, just maybe, you can be wrong about the Bible being the Word of God and still be a good Christian.
2. "I think he would like it to be..."
Well, no one asked you what you think, did they? Maybe he has no preference one way or another and is just waiting for you to make a plausible case for your thesis.
3. "... but is overly obstinate and has an awful attitude towards God and his plan for man's redemption."
A lot to unpack there. Has it occurred to you that maybe YOU're the one being "obstinate" with an "attitude" that won't budge no matter how many facts he presents to counter your preconceived notion that the Bible is the Word of God? Like, maybe YOU're the stubborn one, not him? Because he shows you the Bible, and you start making excuses. Oh, that's the Old Testament. God's different now. He's really kind and gentle. He did what he did before because he HAD to to fulfill the plan of redemption.
Problem: The plan of redemption is only the plan of redemption because God wanted it that way. It didn't have to be. He could just accept an apology without shrugging his shoulders and saying oh well because someone found a particular fruit of a particular tree to yummy to pass up (He also could have put that tree ANYWHERE ON THE PLANET but instead put it right in front of two people who did not know good and evil; then said don't eat from that tree. Not exactly a strong case for omniscience. It's like I put a cookie on the table in front of my 7-year-old and said "Don't eat that," then walked out of the room. He's gonna eat the cookie. I'm not all knowing, and I know that).
So your friend, I submit, is not stubborn. Rather, he's amused at the contortions you'll twist yourself into to deny what's obviously written.
There IS not idiom of permission in the Bible. Bullinger, for what he's worth, appears to be the only one who makes an issue of it. It's hardly a scholarly consensus.
The existence of other figures of speech does not verify the "idiom of permission" as something the Bible employs on a regular basis.
It is, however, an extraordinarily convenient tool for believers to employ whenever their holy book shows God doing what no good God would ever do, even though the book is unambiguous about it being God who did it.
But that's just the old testament. Unless, of course, you're holding back tithes from the apostles in Acts, which is New Testament. (Oh, but it doesn't say God did that. It was Satan -- even though the Bible doesn't say THAT either).
The Bible is filled with examples of God saying he'll do something and then saying He did it. It doesn't say he allowed it to happen or he allowed Satan to do it. It says HE did it. Now, it COULD have said he allowed Satan to do it, very easily. Look at Job. Satan did those things. It says so. Yeah, he got God's permission, but it says that, clearly. There's no ambiguity, and there's no "this is how it works normally."
A figure of speech is supposed to be a statement that is true in essence though not literally true. "It's raining cats and dogs" is a figure of speech. "This car can stop on a dime" is a figure of speech.
A figure of speech is not supposed to be a way for you to get the Bible to say the opposite of what it clearly says just because what it clearly says is inconvenient for your theology. God ordered the execution of a man for picking up sticks on the sabbath. He didn't give man permission to kill the offending sabbath breaker. He gave man an order -- cast those stones!
God didn't allow divorce. He prescribed it. He didn't allow Satan to kill all the firstborn of Egypt. He had it done.
And he DID have a choice. When my kid offends me, I have a choice how to discipline him. You have no idea how many times my discipline has stopped short of killing him because he did his chores between sunset on Friday night and Saturday night!
So here's a thought. Bear with me: Maybe your friend isn't the stubborn one in this equation. Maybe he's not the one being inflexible. Maybe, just maybe, he's given this far more thought than you have.