Think your questions here are important ones. I'm very curious if the second one stopped cooperating with SAPOL before or right after the firearms arrest.
From cases I've followed, it doesn't seem very uncommon for the partner of someone suspected of a crime who was initially helpful to LE to then stop helping and take the 'side' of that partner being investigated against police, particularly as the investigation ramps up enough to start feeling harassed. A lot of the time it seems to be feeling sorry for the partner ('they're all alone,' 'no one else believes them,' 'they need me,' 'the cops are picking on them,' 'it's not fair'), either believing innocence ('they could never') or feeling guilty, like it's a betrayal, for doubting innocence (or being made to feel guilty for it; 'You know me better than anyone, how could you believe I could ever kill someone?' paraphrasing many a killer), loyalty, as you say, getting sucked back into the 'we're a team, we're in this together, it's us against the world' mindset. Feelings of love, trust, history, loyalty, obligation don't just switch off when someone you have deep connections with is suspected of a terrible crime, they're both powerful forces that drive decisions all on their own and also things that can be actively manipulated by others.
But if this arrest was used as an opportunity to separate and question them, and the way it played out ended in the second one ending cooperation, it's possible something in the course of it made the previously cooperative non-suspect (or their lawyer) realize they were beginning to be implicated in this too, if they helped in a cover up or something of that nature. Or if somehow wholly innocent and genuinely in the dark as to what happened to Gus, possibly been shocked by the implications of the questions being asked, didn't believe it was possible the other could do what the police were suggesting, went into denial, whatever. Or even just realized that their own answers were beginning to make the suspect look more guilty, despite maybe initially believing that cooperating would help prove innocence. And any of those things could have been the push needed to comply with what the suspect probably already wanted, which was for them to help stonewall SAPOL, be a united front giving them nothing more than they already have.
JMO.
