Hi there!
Guess what? If your lady is taking her pills correctly then there's no need to worry. Same thing just happened to me, literally yesterday. Supposed to get my period, didn't, but it came eventually...
A lot of things can change when a period comes... Stress, worrying too much, illness... A whole bunch of things! Even if you're on the pill for years you can still become irregular every now and then (you should take a look at the "slightly concerned" post from yesterday).
Just wait it out, and if she doesn't get it, don't panic... She can take a pregnancy test if you need to reassure yourself some more. I would say, if she doesn't get it next month either, maybe she should call her doctor and see whats up. Hope that helps some!

She took an ept today and it turned up negative so I guess we're ok. However, I went and looked at the test again after about and hour (it was sitting in the trash), the negative mark is still there but if I look veeeeery closely and hold the test at a certain angle to the light I can see a very faint outline of the positive line. I'm a little more worried now. Maybe am I just seeing the "testing strip" area? It's not shaded at all, I can just make it out at that one particular angle. Looking at it straight on there's nothing. Also, I looked at the test about 10 min after she took it and there was nothing then.

She did another ept this morning and it was also negative, so we're both feeling much better now. I guess maybe this is just one of those random missed periods while on the pill.

Boy have I been there!! Same thing has happened to me, took a hpt and was negative. Sometimes after being on the pill for such a long period of time, it's normal to not get a period. You guys should be fine!!