Posting for a friend - dodgy batch of One Step HPTs?

A friend of mine is trying to conceive and AF was due last weekend. She did a One Step hpt and there was a definite faint line. She ordered more tests and has done 2 and there's no line at all. Still no sign of AF. Could this be a faulty batch of One Step tests? She's got a couple of FRERs as well so will try one tomorrow if AF hasn't shown. Would love to hear what you think!