Hahaha Dumas that was funny hehe
I agree with everyone
Get a vet out to see exactly what is causing the lameness and what are ways to correct it (if any ways). There could be many reasons that a horse is lame without having the owner being too heavy as the reason.
Really soft soles (and stepping on something pointy like a rock or gravel)
Joint problem (like athritis)
A farrier can tell you if the horse is lame, but I'd only trust a diagnosis to a vet that knows what to look for and knows his/her stuff.
If the vet thinks that it is caused by the rider being too heavy, get it written down and give the report to the owner that way they'd be more upset with the vet for saying they are too heavy than if you told them.
I'm sure if someone told you you are wayy too heavy for the horse your riding you'd kinda feel hurt, especially if you love the horse and have bonded with her...don't put yourself in the position of being the "bad guy"
But it's great that you are thinking of the horse! Kudos for that