"STAY OUT!"
They created their own problems. They screwed up their marriage, they screwed up their child, & it'll screw you up too if you get involved.
Stay out of it. Ask them to not talk about the subject to you because it upsets you, if that's what you need to do. You'll end up being a scapegoat for one, or both, & the child will end up hating you for interfering.
Distance yourself from this, or you will be cast down into the mire.
The child is obviously acting up because of all the turmoil in his parents lives. The parents try to ease their own concience by trying to be his best friend instead of being his parent. He's got no boundaries, no roll model, no stability.
If a member of my family were to go through this, I think the child is the only one I would offer an open door to. My home would be a place of sanctuary & sanity for the innocent - as it is now for a few of my childrens friends. Here he could fine purpose, value in himself, peace, & guidance with a firm hand. He would have to adhere to rules, be part of the family unit that would depend on him for his efforts, & he could depend on us for our support as long as he was not destructive to himself or others.
One of my daughters friends was getting especially bad, starting into drugs & the gang culture. I bought an old Mazda RX-7 for $300 & asked him to help me fix it up when my vacation time started - of course I offered to pay him when the job was done. I spent my entire three weeks vacation with this kid, rough at first (send him away once because he was high & I couldn't trust being under a car when my partner was high), but we came to understand each other. The car was running after two weeks, I paid him & told him he could have it if he kept a job for six months. He was working two days later, came to work on the car every night with me to work out the fine tuning & aesthetics for the last week of my vacation. The night before I went back to work, I handed him the keys & let him know the title would be available when his obligation was done.
He has since been given the title, gone to community college, has a good full time job & wants to continue school. A month ago he sold the car & bought a new truck. He even came by to offer the car to me before he went truck hunting. I told him that I was proud of him & he deserved every bit of the benefit from the car. A great moment for us both.
To this day he goes home only to sleep & I have never met his parents.