Tell 'em if they don't like it, don't come over. I'm sure that you keep the pee pads changed frequently, it's not like you let the poo and pee sit there
They'd really hate my house, cuz I have a pee pad in the KITCHEN
*gasp*
It's changed when it's needed and it never smells in my house at ALL, so I know that with you having only one dog, it can't be stinky
Just ignore them, or make fun of one of their "things" that they do