It's better for her to take them in the morning when she first wakes up. Hcg, the pregnancy hormone, tends to peak overnight and in the early morning, so you're mostly like to get a strong reading then. It could be lower in the afternoon or it could be she drank more and her urine was more dilute. Different brands of tests can also give different readings, so you can be negative on one but positive on another, depending on sensitivity. And yes, unfortunately it's possible to get an early positive that fades and goes away. It happens when baby tries to implant, but something goes wrong and implantation isn't successful. So your body makes the hormones, but then stops as unfortunately it isn't a viable pregnancy. Most ladies who experience this will just go on to get a normal period when they always would. It's also possible she just got a couple bad faulty tests, which does happen sometimes.
Do you have any photos of the tests? Someone here might be able to take a look at them and tell you if they see anything you might be missing. I would try another test tomorrow morning though and see what happens.