I know it seams harsh but maybe telling her would be the best thing for her. If she really loves riding and wants to ride then maybe being told that she's too big for your horse might motivate her?
Perhaps if you word it as ' because of his/her injury and age (horse's name) can only carry a maximum of ...'.
I ride as part of a university club and unless you are in the advanced class (which is maybe 5 out of 20+ people) if you are over about 10 stone (64kg) there are only three horses you can ride. And they simply say you can't ride Marco (or whoever) because he only carrys up to X-amount of weight.