I am fairly certain your friend's g-grandmother was a Rigby, not a Rigsby. But there are some problematic southern families who show up as Rigby sometimes and Rigsby at other times, so it's hard to say for certain.
The 1880 census entry you cite shows Stephen as Rigby. The Civil War records at Ancestry.com also show Rigby. Given that Jesse is also shown as Rigby in the land and census records, I think we can say Stephen at least was not a Rigsby.
The question then is whether the children listed are his and thus Rigby, or from an earlier marriage of Nancy to a William Rigsby (or Rigby).
I'm fairly certain the William L Rigsby of the Civil War was this William Lloyd:
He did indeed live in Randolph Co AL up through 1860. After that he moved to Coweta Co GA.
The other Rigsbys who bought land in Randolph Co were from the same family, descendants of R35 Allen Rigsby 1770. Those families are well-documented, and I can say that your friend's line is not from them.
Your William also is not the William in Cobb Co GA on the 1850 census. That William is a cousin of William Lloyd. The William of Cobb Co moved to TX before 1870.
The family of the Susan Rigsby who is in Dallas Co on the 1840 census matches the family of the E Rigby there on the 1830. I'm not sure who E is. I thought he was R34 Enoch Rigby/Rigsby, who was in Dallas Co for the 1820 state census. But it really looks like Enoch moved to Houston Co GA, and then to Randolph Co GA.
The Elijah Rigby you mention brings up a good point, though. There is an Elijah Rigby in Monroe Co AL (just south of Wilcox) on the 1860 census. He is 62, and living with a Jesse Rigby age 68, apparently his brother. He almost matches the E Rigby in Dallas Co in 1830, who is shown as age 20-30. Jesse, though, matches Stephen's father very well. I had assumed that E Rigby had died, since Susan was listed as head-of-household in 1840. However, now it looks like they divorced. Elijah Rigby is in Dallas Co for the 1840 census, living alone. (I don't know how I missed him before this.) He is also there on the 1850 census, still Rigby and still living alone.
I think the most likely scenario is this: Your William was son of Elijah Rigby + Susan. He married Nancy and served in the Civil War and died as in family tradition. Nancy then married his cousin Stephen Rigby. The question still remains whether William died in 1873 or 1875, and thus whether Elizabeth's father is William or Stephen.
It looks like Elijah and Jesse were really Rigby, and the few records with them as Rigsby were mistakes. I don't know who their parents could be, but there is a very good chance they are related to the Jane Rigby age 80-90 who is on the page after E Rigby on the 1830 census.
Do you have records that show your friend's ancestor as Rigsby, with the s?