Document document document. Things might be fresh in your mind right now, but if you can write everything down with clarity and simplicity about what you saw, what you supposed you saw, etc, and keep an ongoing record like that, then later if shit really hits the fan, you’ll have solid testimony about what you did and did not see. I am not a lawyer, but it seems like your record of “his parents threatened him on monday night” combined with “he was taken to the hospital tuesday morning having “fallen down the stairs”” is the kind of dot-linking that is going to need to happen to get your friend out of that situation. Also, talk to your parents about your suspicions. If you are old enough to post on Fluther coherently, you are old enough to say “mom, dad, I have something important to talk about. My friend, xyz, I think his foster parents are abusing him” And if you don’t trust your parents, maybe you and your friend have a mutual teacher at school. Talk to them about it too. And bring your written down evidence to all of this.