Obviously, no one here can tell you whether or not she's pregnant. My boyfriend and I have been where the two of you are.. Trust me, I know how excrutiatingly worrisome it can be, and I totally sympathize.
However, hpts are pretty reliable, and i'd wager that if she took two of them and they both were negative... She's probably not pregnant. Especially if she took them this long after she was expected to start.
The delayed period could be from any number of things, but it's most likely because she's stressed out; whether it's about this or something else or a lot of things together. I'm not going to tell you to tell her to relax, because it will never work, I know. However, if she (and you) is that worried, she should go get a blood test. It can at least give her peace of mind and maybe help her relax.