Leave them alone.
It's none of your business, you don't know if what you're hearing is the truth, and quite frankly, calling the state will only serve to separate them (at worst) or create tension between them.
Instead: Pray for them. Be an example to them. Spend time getting to know the parents. Get to know their challenges and help them to meet them. And be a good role model.
When I hear that your daughter's best friend is a boy, and is "very close to you" it sends of all kinds of red flags in my head. Not because it sounds like there is anything going on between YOU and him, but because that says something about what you value as a parent.
It's clearly important to you that you be your daughters' friend and confidante. (whereas another parent might not choose that role in the same capacity) and because you are the "cool mom anyone can talk to" in that scenario, you are bound to hear all kinds of things, some true, some not, some exagerrated, etc.
You say he's part of you family and that you have had a great part in raising him. But you aren't his parents, so the best thing here is to help his parents by getting to know them and having lots of patience and compassion and kindness for them to help them figure it out, not for YOU to try to replace his parents in his mind. You are not his mother.
Whatever it is , (And I'm assuming it's far worse than whatever you described above, because that was so very minimal in the big scheme of things) it can be overcome by love and kindness. I'm willing to bet if he's confiding in you, he is NOT confiding in his parents, which is further stressing them and alienating him from them. You will do him a greater favor by teaching him to respect and honor his parents, to listen to them, to obey them, etc (and thereby to repair their relationship.) than to "dwell" on some isolated incident and make a mountain out of a bad situation.