I agree that more info is needed.
I have noticed though, that a lot of women take their partners watching porn as a sexual slight. Especially if he chooses to watch porn rather than have sex. But there are a lot of reasons a man may do this that has nothing to do with his partner. Erectile dysfunction, fetishism, fantasy, other sexual issues, or simply needing alone time, are all good reasons.
Its true though that sometimes people fall too far down the rabbit hole and develop problems. My brother began watching hentai at about 8 years old, and having aspergers on top of everything caused him a few issues over the years. He was very honest about it. He said that he's tried to cut down or quit, but he had a loss of control over how long or how offen he'd watch it. He said that he'd become preoccupied with it, and that he spent a lot of time and effort in accessing it. That he had to step up to more extreme stuff to be satisfied. He even gave up social and recreational activities in order to spend more time watching it. Substitute porn for drugs and you have symptoms to justify a severe chemical dependence.
Not only that, but the nature of addiction is that it's connected to your sense of pleasure, and it becomes difficult to experience pleasure without it. It also warps your priorities to put the addiction first, and all else second.
So I think it's safe to say that porn addiction is as possible as food addiction and other non-chemical addictions that play off our basic instincts. After all, our brains are more than capable of producing it's own chemicals to support just about any habit.
Anyway, if your brother-in-law's symptoms are along those lines, then I don't know what can be done. As has been said before, addicts can't be cured, only supported in recovery. It's not something that every spouse is willing to go through.
Also, as has been said, happy, healthy people don't just wake up one day resolved to develop an addiction. If the issues that got him to this point aren't resolved (they may have nothing at all to do with sex), then there's a very high chance of relapse.