My seven year-old has cerebral palsy, so I've been on the other side of the coin. Mostly what I remember about those difficult first few months was that a lot of people started thinking they had to either make grand gestures or none at all. They planned special birthday parties for us in the hospital, just for us, but then didn't invite us to their "normal" birthday parties. They would offer to come over to clean our house or bring us dinner, but not just come to "hang out." It's not that the grand gestures are unwelcome. Far from it. It's just that you need the normal stuff too.
Another thing that can be very overwhelming is answering the same difficult questions about your child's condition over and over. A lot of parents deal with this by using social media of some sort to post the general updates. If your friends do this, try to respect it and not ask questions that are answered there. We know you mean well, and you personally may not have talked to us for a few days, but when we get several phone calls a day asking us to repeat the same information, it can get overwhelming. What is okay is saying something like, "I saw on twitter the surgery/therapy/whatever went well. That's great!" If they are up to elaborating, they will.
Another thing to avoid is treating the child like he is ultra fragile. There might be certain behaviors to avoid around him, which his parents will tell you about, but for the most part you can treat him like any other kid. Play with him, talk to him, hold him. With a 175 pound powered wheelchair and a two-year old temper in a seven year-old body, we're usually more concerned with our daughter accidentally hurting someone else than people accidentally hurting her.
Also keep in mind that your friends are grieving for the loss of the healthy child they were expecting. The emotions are not unlike grieving for the death of a loved one. Keep that in mind if your friends seem a little irrational. However, don't treat them like their child died. I know that's a subtle distinction, but you'd be surprised how many people still use their hushed "funeral voice" around us, and avoid making eye contact with our daughter. They still have a child they love very much and who in some ways will be difficult, but in other ways be more of a joy than they could have previously imagined.
Just the fact that you are asking this question tells me you will be fine. Don't be afraid to ask your friends how they would like you to handle certain situations. My advice above is fairly typical from talking with other parents, but everyone is different. I'm never offended by someone asking something like, "Would you like some visitors while you're in the hospital, or do you prefer some time alone together to recuperate?" By all means, don't let not being sure how to approach a situation drive you to indecision and inaction. It's always better to ask if you're not sure.