If I were you, I would lunge him myself in the round pen. Get him to trot for you on the ground. It seems since he listens to her, and not you, you need a better bond with him and show him you are the leader.
If he gets aggitated when asked to trot, perhaps it is him being lazy but also there might be another cause. What kind of bit are you using on him? Do you tense up when you ask for a trot? Do you tighten the reins and grip to him for dear life? Was he previously smacked or kicked? Are you using spurs?
Lettin' the cat outta the bag is a whole lot easier than puttin' it back. -- Unkown