So a lot of this is going to be contextual - how important the friendship is, how deep he has gone into the manosphere, how long he's been in it etc.

I'm going to approach this from the assumption that it's a long-game situation and that you care about the person deeply. Pretty much everything applies from this but whether you choose to maintain the friendship or whether you decide to end the friendship or you aren't willing to invest as much into this project as it demands is your prerogative.

Basically in a long-game situation your primary concern will be to always maintain the relationship and lines of communication. If you don't have those two fundamental factors, you will be unable to effect any change.

What this means is that you will almost certainly need to be judicious in what you choose to push back on and when you decide to do it. What this looks like, in practice, is letting things slide by if they do not serve your overall goals. I'm not saying that you have to tacitly or even implicitly support their opinions but if you are skilful about it you can make asides to voice dissent without dragging something down into a debate. Throwaway lines like "I don't really see it that way" or "That doesn't track with my experience" before carrying on the conversation are going to be important here.

Your friend has almost certainly taken the trauma of a breakup and turned it into manosphere bullshit. What this means is he likely feels lost, powerless, abandoned, disillusioned etc. and the manosphere narratives are assuaging these underlying feelings. You will need to approach your interactions with him in a way that does not threaten him or aggravate these feelings of powerlessness etc. because if you position yourself as a threat to the beliefs which give him a sense of security and power then you will aggravate the underlying causes for him falling to the manosphere and you will almost certainly make him dig deeper into the manosphere as a way of bolstering himself.

You will need to walk the tightrope of being a friend to him while not being an ally to his beliefs. You will have to demonstrate that you will not abandon him and that you are not going to force him into positions where he feels powerless. But at the same time, you cannot endorse his beliefs and you will need to get him to trust you enough that he expresses these opinions to you and then to trust you enough to let you explore these opinions in regards to validity, consequences, implications etc.

This is where the real work takes place. You need to be delicate and engaged while also holding a position of detachment - if you treat these discussions where you explore his beliefs from an antagonistic angle or where you are heavily invested in it emotionally, it's going to result in arguments and shutting down and similar counterproductive outcomes.

Essentially, you want to get him to move from a totalising narrative such as "All women are b*tches" to something which has nuance, even if it isn't a complete reversal. This might mean that when he says something like this and you have decided that it's appropriate to challenge it in that moment, you could reflect that he doesn't treat his mom/sister/etc. as if that statement is true. Then you want to explore this apparent contradiction and use dialogue to open up space to compare, reflect, challenge, and further explore.

If, over time, he moves from "All women are b*tches" to something like "Most women are..." or "Women can be..." then that's progress, even if it doesn't feel that way.

Keep on chipping away at these values by exploring them, gently countering them (especially with real-world examples), and ultimately getting him to question the narratives himself.

It's kinda hard to give a clear procedural roadmap to how you would go about challenging someone's beliefs because it's all so contextual but I hope this is a starting point for you. And I just want to give you a caution that if you approach interactions with your friend from the position of "I'm right and he's wrong, he needs to learn from me so that he can see my point of view and why I'm right", you're never going to make progress. You have to be humble, open, curious, and most of all gentle.

Good luck with it.