How can you care for someone in an anonymous virtual community when their behavior changes in worrying ways (see details)?

Suppose a person you knew informally (in a virtual community) started exhibiting strange behavior. Perhaps they were more combative than they ever had been in the past. Perhaps they start to misunderstand things regularly, whereas in the past they had never had troubles in the cognitive area. Suppose this started happening more and more regularly. You start to wonder if this person is starting to experience some form of dementia—perhaps Alzheimer’s or early symptoms of some other organic change in their brain.

What would you do, if anything? It’s an anonymous virtual community, so you don’t even know who the person really is. Would you address it directly with them? Would you try to find out if the person had relatives and then try to talk to them? Would you just ignore it because really, there is nothing you can do?