Based on how you've phrased your comment, somewhere along the line a person or group of people convinced your friend that these religious extremist views were righteous and helped to foster that relationship with him that lead him further down (or even into) that path. He wasn't born a religious extremist, in your own words "he became a religious extremist".

The truth is, it sounds like you're asking for a solution to a specific situation, but a random internet stranger with no real insight into that situation, no knowledge of that extremist's perspective and experience -- it's literally not possible for me to help you understand beyond very broad generalizations.

However ...

People can change their minds. Maybe not everyone, but what's certain is that they absolutely won't if there's no path that allows them to retreat from their extremist views/past when the call comes. Accountability may be necessary, but that opportunity to abandon old ideas needs to be clear of obstacles for the best chance of success at the start.

It might help to realize that you personally may never be able to change that one specific extremist's views. And even if you could, you might not be around at the right place and time to accomplish it. Focusing on that one extremist, in my opinion, is probably not the healthiest or most viable strategy.

No way I'd have the time or motivation to write everything I'd like to say to you, but I'll leave you with one anecdote (details intentionally obfuscated because the last thing I need is to be tracked down by 10th degree black belt sleuths with a penchant for extreme violence).

There was a regular counter protester who liked to come to protests for "liberal causes" and confront random folks who were showing their signs. He was actually getting to know those folks, seeing that they were nice & decent people unlike what his media bubble suggested, he got to see that they all had differences of opinions but chose to focus on what they all agreed on, etc.

Obviously everybody clapped and unicorns farted pots of gold, but the counter protester chilled the hell out significantly over time. He's told various people at various times that he actually respects what the protesters are doing even though he disagrees on the points. I haven't seen it, but I've been told he even responds to some of the most vile comments online in support of what those protesters are doing. He's distanced himself from the MAGA bullshit and Trump support.

The thing is, nobody sought him out, specifically, to change his mind. He's the one who showed up. He didn't turn into a card carrying registered Democrat liberal hippy. But we take the wins when and where we can get them.