The main character, Ponyboy, might be shocked at first but then try to be understanding. He's a sensitive soul, so he could offer Sandy emotional support. Darry, on the other hand, might be more practical and think about how to deal with the situation in terms of finances and family reputation. Sodapop could be a bit more conflicted, feeling sad for Sandy but also worried about how it affects the gang.
The other characters' reactions could vary. Some of the more hot - headed gang members might initially be impatient about Ponyboy being sick and not being able to participate in their activities. But as they see how serious it is, they would come around. The Socs, if they knew, might not care much on the surface, but there could be a few who feel a bit of sympathy, especially if they start to see Ponyboy as more than just an 'outsider'. And the girls in the story, if there are any, would probably be very motherly and bring things like soup and warm blankets for Ponyboy.
The outsiders in the novel could be characters who don't fit into the main social or cultural groups depicted. Maybe they have different beliefs, backgrounds, or behaviors.