(I want to clarify what we mean when we say different distances. Do you mean to say that Quillathe is just inside your 20-ft. reach, so there are 3 empty 5-ft. squares between them? Or that she is 15-ft. away from you, so there are 2 empty 5-ft. squares between them? I interpret it the second way.
Also, I take it you're not hastened this round, so no AC bonus from that?)
Quillathe spring attacks at the giant dwarf, attacking with her bladed whip, still maintaining her 5-pt. Combat Expertise. Since her speed is 40 ft., she moves in, attacks, then moves past Burgeddin, heading to the tree line. She ends up just 5 ft. from the tree line. Spring Attack keeps her from provoking attacks of opportunity.
Her attack roll is d20 +20 -5.
[dice]