I am so sorry for your friend's family. It must be absolutely horrible for them. You are a good friend for wanting to help.
Since you were emailed the news, start by replying via email - directly to them only, not to the whole group. Just express your sympathy and say that you know it must be a very hard time for them. Continue offering prayer if it has comforted them in the past. Don't mention that it is God's plan, or that they will get through it, etc. Just express sympathy.
Offering to bring a meal is still a huge help. Be specific - "I'd love to bring you a meal next week. Is Wednesday or Thursday better for you?" Or whatever choice of days is best for you, of course. Don't leave it open ended, as they will be overwhelmed with offers to help and probably can't reply to them all.
Remember that they could be dealing with this for 20 years. Don't forget about them a year from now, or five years from now. Stay in their lives as someone they can count on to help when needed. Times when their son is particularly sick will be when they need the most help, but also when it might be hardest for them to ask or recognize their needs. So continue to offer over time.
If they have other children, offer to be an emergency on-call babysitter. Let them know they can call at 3 a.m. if they have to take their son to the hospital and that you will head straight over to watch their other kids. Offer to have a regular day each week that you watch the other kid - or the sick one if you are capable of meeting his needs - so she can have a little break.
I'm sorry you're going through this and I hope that, however long he lives, their lives are happy.