Is it permissible to change the password of an email account containing pornographic material belonging to a deceased person, with the intention of preventing his friends from viewing it and thereby relieving the deceased of its burden, and would this action be considered an infringement on something to which I have no right?

What you intend to do is not theft, but rather an obligation, and it is considered an act of changing evil, which the Prophet, peace and blessings be upon him, commanded when he said: "Whoever among you sees an evil, let him change it with his hand; if he cannot, then with his tongue; if he cannot, then with his heart—and that is the weakest of faith." Narrated by Muslim. This is in addition to showing kindness to the deceased by stopping what incurs sin upon them.