Why don't you tell her the truth, then support her weight loss? Maybe you can help her work-out in lieu of lessons, make the weight-loss horse themed with lunges, maybe she can jog alongside the horse, be outdoors working out, running wheelbarrows, carrying buckets, cleaning stalls, and the goal is if she loses x amount of pounds, she can ride and it will be a celebratory event! They did it at my old barn. The instructor would have an equine fitness class for her students 2x a week. They would lay out in the pasture with their yoga mats and work out on exercises for their wellbeing and for helping them ride better.