The one thing you can do is to stay calm and be quietly supportive without taking sides. If you do help them with kindness/anything practical and then find out they were guilty, it's not the end of the world- it's not as if you were aiding and abetting them.
On the one side, you can't really know that your friends are innocent, however well you think you know them. Parents do hurt their children, even parents that noone would suspect for a moment. And yes, sometimes they do take them to hospital afterwards. Not everybody who hurts their child meant to do it, and if you are a loving parent and discover you have hurt your child- yes, then you would try to get them to hospital.
It may also have been an accident (like falling with the baby)and they are too scared to say. I put dd's pram together the wrong way when she was a baby and it came apart in the road- she went head first onto the pavement. Thankfully, she wasn't hurt, but she could have been. I like to think that I would have had the courage to tell the truth at A&E, but I can see that a parent might panic, particularly if the fall is a result of someone being slightly careless (like leaving child unattended on changing table).
On the other side, I think it may be overly optimistic to suggest that if there was a medical condition, the hospital would have picked up on it. That all depends on the competence, experience and preconceived ideas of whoever saw the baby at the time. Children (and adults) are misdiagnosed/not diagnosed when they should be every year.
I myself had a very strange experience of dd being kept in hospital and not allowed home. The consultant paeditrician thought she must have been sexually abused because she had ankle pains despite X-rays revealing a lack of broken bones. To his mind this meant she must have gone through a trauma= sexual abuse. And this was a 7yo who was perfectly capable of explaining her own situation. Of course, it's easy to say that this couldn't have happened, that any doctor would know that the human body is not made up of bones alone, so no hospital could possibly have made such a mistake. Except they did. And I was terrified.
Children have been taken into care before now, only to be diagnosed later with brittle bone disease; it can take a long time to diagnose.
So both scenarios are perfectly possible. The parents may be guilty or the hospital may have missed something. I think there are two things you can do without committing yourself too far:
you can do the ordinary neighbourly cups-of-tea, offering practical help, offering a shoulder to cry on.
you can suggest that they get legal representation. Even if they're guilty, they still have a right to have their case put.