I am new to this position and I don't think this qualifies as a status change. I believe the participant is still an employee while she's out on STD.

Treas. Reg. 1.125-4 (iii) Employment Status. "; a commencement of or return from an unpaid leave of absence;" is listed as a vaild change of status event.
Whether the employee can make a change depends on your plan documents say and the change the employee wishes to make must be consistent with the event that occured.