Hi, M.. I'm so glad that you have a heart to reach out to this person and let him know that you care. #1, don't stop doing that. It's the right thing to do. #2, it's always appropriate to help with food, even if your friend seems to have a ton of family around. At some point, they really will need someone to pitch in and help with practical stuff. As your friend becomes more and more ill (unless he has a miraculous remission, which I wouldn't rule out if he has people praying for him), his family wiill become more and busy doing more and more things to see to his immediate care, and they will have less and less time for cooking, etc. So be sensitive to the process they are going through, and when they need something like a casserole dinner brought in, don't hesitate.
I don't know how close you are to the family, but it's also a big help to those who are caring for your friend if they can get a break from his care. Sometimes being around a person who is terminal can be extremely nerve wracking and taxing; if you volunteer to sit with your friend and give his wife or other helpers a break, to get some fresh air or just some quiet time, that is an extremely loving and helpful thing to do. It all depends on how close a friend you are, and how close you are to the rest of the family.
Then again, maybe what you're doing now is exactly enough, and you don't need to do any more. The key is to be ready and aware, sensitive to what the Holy Spirit prompts you to offer. It always feels like we're not doing enough to help someone in times like these, but don't let guilt drive you. It's not your fault that he's sick, and it's not your fault that the family is going through this trial. Keep praying for your friend and for his family, and pray that God will use the situation to His glory. Your prayer may open the way for folks to know the Lord; the family may find strength together that they never had before.
I hope that you find peace of mind in what you're doing for this friend, and that God blesses your kind efforts.