Another suggestion you might consider...Towns County was part of Union until after 1850. For instance, Old Union Cemetery was in Union County. It is now in Towns County. George and Rebecca Plott are buried there. With the counties being changed several times, you might have them try Union County's courthouse. Depends on the year you are researching.
I also have an old book from Union that I will check. It is supposed to be all Union County info but there might be something in there.
In the library at Young Harris is a LOT of info gathered by Jerry Taylor. More than I could look at in 3-4 days. I even had my Mother helping me and we ran out of time. Plott info galore in the collection. I would really have them start at Young Harris before heading to Union County or Marietta.