The pattern of travel involves the following:
She can only travel to states with where the trailing letter of the current state's capital matches the first letter of destination state capital.
As such, I believe your cousin has currently found herself in:
Tennessee (Nashville)
Here's the explanation of her trip to Washington:
writing it nowGeorgia (Atlanta) - 5 in: Augusta, Columbia, Helena, Olympia, Topeka. 4 out: Albany, Annapolis, Augusta, Austin
Maine (Augusta) - 5 in: Atlanta, Columbia, Helena, Olympia, Topeka. 4 out: Albany, Annapolis, Atlanta, Austin
Maryland (Annapolis) - 6 in: Augusta, Atlanta, Columbia, Helena, Olympia, Topeka. 6 out: Sacramento, St. Paul, Salem, Salt Lake City, Santa Fe, Springfield
Sacramento is a shortcut
Illinois (Springfield) - 4 in: Annapolis, Columbus, Des Moines, Indianapolis. 3 out: Denver, Des Moines, Dover
Dover is another shortcut
Colorado (Denver) - 4 in: Springfield, Richmond, Concord, Hartford. 2 out: Raleigh, Richmond
Virginia (Richmond) - 3 in: Denver, Dover, Montpelier. 3 out: Denver, Dover, Des Moines
Delaware (Dover) - 4 in: Concord, Hartford, Richmond, Springfield, 2 out: Raleigh, Richmond
Here's the mistake. It should be North Carolina instead of South Carolina
North Carolina (Raleigh) - 3 in: Dover, Denver, Montpelier. 4 out: Harrisburg, Hartford, Helena, Honolulu
Connecticut (Hartford) - 1 in: Raleigh, 3 out: Denver, Des Moines, Dover
Iowa (Des Moines) - 4 in: Concord, Hartford, Richmond, Springfield. 6 out: Sacramento, St. Paul, Salem, Salt Lake City, Santa Fe, Springfield
California (Sacramento) - 4 in: Annapolis, Columbus, Des Moines, Indianapolis. 2 out: Oklahoma City, Olympia
Washington (Olympia) - 1 in: Sacramento. 5 out: Albany, Annapolis, Atlanta, Augusta, Austin