This name is a synonym of Symphyotrichum spathulatum (Lindl.) G.L.Nesom.
The record derives from TICA
(data supplied on 2012-02-11)
which reports it as a synonym (record 64EDC7C2-3AC3-46FF-9BEC-FE2625710330).
This name record (from TICA) was not included in version 1 of The Plant List. It is possible the same name was included from a different data source, to check this you can search for Aster fremontii in version 1 of The Plant List.