A person willfully ignorant like that would not be my friend any longer. Can neither of you see that this horse is trying her heart out, but is struggling? She looks out in the SI as well. This horse needs to not be ridden until her weight is up. And even then, your friend may be too big for this mare's frame. And that saddle is awful on her, it's causing her pain.
You are encouraging her. You are riding the poor mare as well. You are not standing up for this poor animal's well-being. You are helping perpetuate this.
Please, give your head a shake.