I need advice about a TERRIBLY awkward situation!?

Just tell her that you the horses you have aren't really supposed to be ridden by beginners. You can make some dumb excuse, like they are too crazy or not tame enough or it will mess up what you're teaching them if you're start leading them again. And you can tell her that you don't give pony rides to adults because ponies are only strong enough fro about 70 lb kids. I don't really know much about horses and I'm guessing neither does she. I bet you could make up any lie and she would believe it. If she asks anybody else any questions they will notice that she is huge and understand why you told her what you did. She will never know. I hope everything works out.