There’s no evidence that he had an accomplice in disposing of the body, and maybe I’ve been watching too many movies, but I think the alleged mistress would help.
To have an affair she must be a deceptive and dishonest person, probably with low self-esteem. She is already used to sneaking around and lying to people to cover for GBC. Possibly she is in love or infatuated with him and does not want to lose him. He may have some power or influence over her – whether having some kind of dirt on her, or just emotional/psychological influence. If the marriage was unhappy then no doubt she would know all the details and sympathise with GBC having to live in this unhappy situation.
If he called her in a panic and said it was an accident and he doesn’t know what to do and he can’t go to jail, who’s going to look after the kids, you have to help me blah blah (we’ve already heard his ‘distressed’ voice so that’s not too hard to imagine). He might even try and make her feel guilty or obligated by suggesting that the affair was what caused the argument in the first place.
By all accounts this wasn't just a casual fling, if you beleive the rumours it had been going on for several years, then ended and apparently started up again recently.
So I think that yes, out of a misguided feeling of love or loyalty, and/or a fear of losing him, she would help him cover it up, or at the very least if she didn’t help physically to relocate the body, she might have advised him what to do and turn a blind eye. In a sick way she might see this as a chance for them to be together, something that would have been unlikely before.
I can’t imagine helping someone to do this either if I got that phone call, but some people do strange things in the name of love.
What’s that saying – friends help you move, real friends help you move bodies.