Kierra moves beside Yorn and Casts a heal spell on him. This invokes an AoO on Kierra. Unless the real Kierra posts then she can do whatever she wants.
The +4 ac and staggered cancel out the +6 to hit.: 1d20 ⇒ 4 Miss
Healing To Yorn: 2d8 + 4 ⇒ (1, 3) + 4 = 8 That roll is crap Yorn you heal 10
instead.
Edit: gonna put Kierra in position to flank for Yorn with her spear. Sneak Attack damage Yorn would have gotten if he waited on Kierra to move and knew about my flanking rules: 2d6 ⇒ (4, 3) = 7