A friend who becomes unpopular overnight is still the same friend. Act like it.

Social weather changes fast, and usually for reasons that have nothing to do with the person. One rumour, one bad moment, and everybody is standing slightly further away. Nothing about who they are changed in that hour. What changed is what it costs you to be seen next to them, which is a completely different question.

Sit with them in public, not only in messages. You do not need to make a statement or defend anything out loud. Keep the friendship exactly where it was and let people watch you keep it there.