I can think of three possible responses. Pick the one that suits you the best and move on.
1. Express sympathy: This is a nice thing to do, especially when a member of a different party has serious health issues. When most people see you doing this, they see the human side of you. This makes them more likely to support you and less likely to oppose you in the future. This is what smart, successful leaders of all ideologies do.
2. Don't say anything: This can be acceptable as well. People get sick all the time and no one has time to comment on each piece of sad news.
3. Express glee at the politician's hardship, or imply that he somehow deserves it. When activists do this, I don't think that it helps their cause. Politicians will do things that should be criticized. They will do things that should be praised. And then they will die, and someone will miss them. You can feel gleeful about cancer and death if you want to. It just strikes me as a bad idea to express that feeling out loud.