A person (a shaykh) told his critical health condition to a brother (in secret, and he doesn't want to be known), and this brother told to someone else so that he would try to help him. Is he sinful for that?

Ustadh AbdulAziz al-Haqqan

disclosing it when a clear benefit for its owner calls for it, and only to the extent needed, while you are sure that it is the case, then you can do so in sha'a Allah. If you are not sure, then do not do it.