As an inquisitor, he has access to the inquisitor talents that lets him spend fortune points to reduce insanity and corruption to zero. So in essence, he wouldn't go crazy or suffer any repercussions. I told him that if he negates the insanity or other ailments gained, he's not accepting the item on full and thus would not get the bonuses. I mean the halo device changes his genetic makeup as he physically changes throughout the stages.

He reluctantly agreed, but I didn't want to be unfair. Wouldn't he still get the described side effects such as vivid nightmares, hallucinations, etc.? He just said he was just so badass as a character to deal with all of that easily, but this isn't some simple tool he's using. This is a halo device. Very ancient tech that no one understands as the knowledge was lost.

Any thoughts on this? Mind you I know I am using old Dark Heresy rules for the Halo Device since I don't have a reference for current edition.

Talk with the player on what they expect out of the Halo Device and what you expect out of it. In-setting, one should stop to pay very close attention to any mention of those things, because as you've noted they're bad news for just about everyone. I can understand that the player's using some late-game mechanics to heavily mitigate its side effects, and while that's cool to see someone pull off it's also not quite in line with the striving-against-the-insanity-of-the-monstrous-universe tone of 40K.

I've included one in my now-ended Black Crusade game, but mostly as a joke on how insane and callous the user was even before using such a device. If my Rogue Trader crew caught wind of one they'd stop what they were doing, haul ass to the source, and lay waste with extreme firepower.

So in essence, he wouldn't go crazy or suffer any repercussions.

Depends how long he keeps it attached for. He'd stay sane....ish.... meaning by sheer force of will he'd stay himself (mentally, at least) - but he'd have to be pretty unhinged to blithely accept voluntarily doing this to himself. You could say the same thing about an individual voluntarily excising his central nervous system, hooking in an MIU and going 'brain in a jar', but this is not unheard of by any means.

I could accept the argument that anyone who knows what it actually entails actually wanting to be an inquisitor must be a bit wierd too.

That's not the same as saying there'll be no consequences, though. Spending a fate point against the 'immediate' instance of corruption or insanity is fine, and I'd be happy to accept that he'd be able to spend a fate point for the 'decades of' events - a game session may be indeterminate length but it's unlikely even the longest one is going to cover ten in-game years.

Thats not the same as saying there'll be no consequences. If he's accepting any of the mechanical benefits (of the unnatural characteristics sort) then it must be because the device is physically changing him. So whilst he's not accruing corruption, he is visibly and obviously mutating, and is likely to receive all the attention this entails.

Secondly, I would argue he won't be able to apply a fate point to reduce corruption or insanity from a resurrection. If you've got yourself bloody well killed, then you shouldn't have any unspent fate points left!

Why in the world would he want a Halo device? Unless his character was insane, or was desperate to prolong his life (in the short term). His talents and skills can mitigate its ability, but on temporarily, as a GM, I would let your player know that if he does use this thing willingly, he is expecting to replace his main PC soon. Those devices are akin to deamon possession.

Highly heretical. Also any inquisitor pure and radical or even his own agents may turn on him and hunt him down as he becomes a deamon, Xeno, or whatever monster it will eventually turn him into.

Halo objects are really cool and interesting, but should only be taken by NPCs or player who wish to retire their character. Or if you're feeling kinda mean, but may make for a good story, don't tell the player what will happen to him to make it more exciting.

My personal house-rule on the various Corruption-reducing things is that all of them have a caveat; if you know the consequences of your actions and willingly do the corrupt deed, you can't prevent said Corruption with Fate Points/abilities. It's hard to resist something when you take it into your arms.