Struvite stone diet

My 8 year old dog is on a special prescription diet for struvite stones. How bad is it if she eats food outside of that? I'm not talking like an every day thing, but if she eats some occasional crumbs my son drops or if she finds a few kibbles of another dog's food at the dog sitter's house. Is that kind of thing enough to give her more stones, or will an occasional slip up be ok?