Quite often my training sessions are watched by other interested parties or the partner of the owner. Just lately this has been a real bonus as they can often point out the differences between how I handle the horse and how the owner does - "you need to be more military when you turn towards the horse" said one man. On another occasion it became clear that while the owner didn't hand feed, her husband did - as soon as he took hold of the lead rein the horse started to mug his pockets! A bit of a giveaway that.....that particular horse didn't mind a bit of inconsistency!
It does help if the partner is supportive and not sceptical. Everyone needs to be on the side of the horse.