Well, if he were pulling that on me, I'd apply hard heel far back on the inside and push his rear end around.
The shoulder in and out are exercises that work on supplenss and leg yielding, following a straight line down the arena wall with the horse canted diagonally to the track. In essence it's a trotting semi-half-pass.
I'll suggest that to her, that may work. She loves him dearly but occassionally, she really gets frustrated with his behavior.
You wonder sometimes what's in their heads - he really enjoys being worked and hitting the road, and you can see him light up when she shows up.