Another player can use the Heal
skill to Grant Second Wind
DC10 to the dying player character without taking an action, assuming they still have their Second Wind
.
The point "without taking an action" is important as the dying player will have the Unconscious
status, so they can't take any actions
UPDATE: see the Heal skill, p.145 of Rules Compendium, Dying status, p.231 and Unconscious, p.234