It wouldn’t be the first time that taking a cycle off ended up working!
I ended up buying some Wondfo bulk tests off of amazon for super cheap. (https://www.amazon.com/Wondfo-Pregnancy-Urine-Strips-Tests/dp/B07CZ5887G). Then I didn’t feel bad about taking them early. They are pretty sensitive too.
The reviews for these things are a little bit biased towards people who have a complaint - the 1 ct pack has better reviews but even FRERS are only around 4 stars.
"True" false positives, aka a test line with colour when somebody has no HCG in their system, are essentially not a thing. Sometimes people can see the indentation from where the test line was put, or it dries a little differently than the rest of the test. Sometimes people have a chemical pregnancy, but by the time they go in for blood tests their HCG is below 5 again.
My pick? Wondfos. They are cheap and work totally fine. If one test is a dud, you take another. Enough to give to any of your friends who are in a panic too. https://www.amazon.com/Wondfo-Pregnancy-Strips-25-count-medical/dp/B0002YIQEQ
This is the workbook I have referred to many a time - I know access for mental health support can be very challenging.
This page has some stuff on proper condom usage. Not using them consistently, touching the wrong side to the tip of the penis and then flipping it, using one that is too small or too big, bad storage, that's all imperfect stuff.
FAM is can be helpful for learning about your body - I never had a 'late' period on it, and seeing my basal body temperature made it clear progesterone was causing my symptoms. I've had great experiences with IUDs, although BCPs were not my friend. If you have bad side effects on something, you can find something else, but most people do find something that works adequately well for themselves. Anyways. Best of luck. :)
I haven't worn jeans in a decade, but I vaguely recall it taking quite a lot of blood to soak through them, definitely more than spotting.
Putting the bleeding aside, sex around 10 days ago, sure take a test. I like the box of strips off Amazon. Very accurate and inexpensive. https://www.amazon.com/dp/B00DOJWJ3U?ref=ppx_pop_mob_ap_share
I think it's totally fine that she does not want to get on birth control, but she needs to do something different because having this level of irrational anxiety every month is just not sustainable. My suggestion is to buy her a copy of Taking Charge of Your Fertility. You can read it together. It will educate her (and you) about how ovulation, pregnancy, cycles, and all that stuff works and maybe make her feel more in control. It teaches a method of fertility awareness that involves taking your temperature every day and paying attention to changes in her cervical fluid, and is very effective (my husband and I use it as birth control) - unlike period trackers, which are almost useless for determining fertile periods. But mostly it's just very educational, and I think she needs some information to make her feel more empowered instead of paranoid all the time, whether of not you want to use the method.