You need to think of the outcome you want from this situation
YANBU to tell her she can't wear them, however will the conclusion from this end up leading to you two falling out over a pair of boots and having an argument?
Sometimes it is better to let things slide for the sake of getting on with your child, children hate being told in sternly that they are explicitly not allowed to do something. You don't want her growing up glad to be eventually moving out because "mum never let me do anything" (of course you let her do things but children hang on to the negatives) or is a pair of boots not worth an argument or falling out over?
Just to offer a different perspective