Perhaps there is some way that you can contribute to the wedding that doesn’t involve exposing yourself to the risk. You could make a list of the things that you can do, whether it’s helping with some aspect of the planning, being available for emotional support by phone call throughout the process, or something else. Then, when you break the news that you won’t be able to serve as best man, let them know that you love (if you love them, I realize not everyone experiences love the same way, not trying to make assumptions)and support them and want them to be happy, and are willing to help in the ways that you can. Make sure you only offer things you are truly comfortable with doing- don’t commit to anything that would be risky for your health. Then, stick to your boundaries. If anyone is upset, let them know that you care about the people getting married. Be supportive in whatever ways are culturally traditional in the social context that you can do, and that you can do without risking your health. Best of luck comrade.