It depends on the whys of the divorce, but tell your friend that the taking sides hurts all parties involved.
My parents were divorced when I was 4 yrs old, and my mother never spoke bad about our dad to us. My dad didn’t speak badly about my mom either. So, if your friend was to be in the middle of parental hisses, tell him to politely ask not to be told anything negative about the other parent.
Another thing that can help is to tell him to keep close to both parents. It’s horrible to be w/o a dad. I lived w/o my dad and knowing he never wanted to know about us, was heart breaking. If your roomy keeps a sane and healthy relationship with both his parents, the easier it will be for him.
Divorce sucks. I am sorry your friend is going thru this.