I disagree with Grimlock77X comment. It's stated in the rules and by Ryuk that the fundamental difference between Humans and Shinigami using the Death Note is that humans don't gain lifespans.
In How to Use: XLII
The use of the Death Note in the human world sometimes affects other humans' lives or shortens their original life span, even though their names are not actually written in the Death Note itself. In these cases, no matter the cause, the god of death sees only the original lifespan and not the shortened lifespan.
^ emphasis added by me
since the rule says it that the use of the Death Note can affect lives or shorten the original life span we can assume that the Death Note can indirectly give a human more life.
think about rape victims who are denied justice (like Yonegoro Nusumi's victim), if cop drama's like Law and Oder: SVU is anything to go by, without counseling their life deteriorates. with Light killed Yonegoro Nusumi or anyone like him the victims feel justice have been served and improve their lives (and there are no future victims for serial offenders) or wont commit suicide from the denied justice.
As for the lifespans, according to this answer Mr X's lifespan would be just wasted so this isn't distributed. there is also no way guarantee the lady would've lived longer or even shorter because of his death, I dare say also that even with Shinigami Eyes you won't see if the lady lives longer since Shinigami will only ever see the original lifespan so likewise a human with the eye deal will see the same, the only way is if you were like Beyond Birthday or L and had worked out how to read Lifespans and knew when the lady would die