I am so confused as to why the moron mechanic - who the author points out is a man - just took random birth control? I have to assume he didn't look at the label and is an opportunisitic junkie and went, "Oh nice, pills! I'mma have me one of them!" thinking it was something illicit/recreational.
I mean if the guy was trying to oops the author by proxy on purpose, I guess he didn't realize that she could just take the next dose? Her placebo week would start a day early, that's it. Assuming she takes the placebo pills and doesn't just skip them. Or he could have taken the whole pack if he was so offended by her attempting to prevent pregnancy. But my money is on the guy is dumb and thought she might take the fun kind of pills and helped himself. Or, as someone in the comments suggested, maybe he thinks he can slip his partner one pill and he can safely bareback her. I know more than one guy who thinks the pill works by you just popping one right before sex and you're covered.
I hope she reports it, but I doubt anything will come of it unless there is video/security evidence showing the dumbass taking the pill. And who knows if the shop would incriminate one of their employees because of something a woman said. The shop and police - were they to get involved - would likely just tell her she must have taken the pill and forgotten, she shouldn't leave pills in the car, or she's just hysterical and imagining things. They could fingerprint the case, but the asshat could just lie and say he had to move it to access (other thing).
Dude is lucky all he took was some fake pregnancy hormones, if he did, in fact, take it himself. He could have wound up very sick if he took something more risky without it being prescribed to him.
I'd be tempted to point it out to the shop and be like, "Yeah I think one of your guys took one of these pills because it wasn't open before. He should probably go to the ER to get his stomach pumped because they're (some made up prescription name) that is very toxic to people who don't have (made up rare disorder) like I do." Let the fucker pay out the ass for a stomach pump and let him sweat. If he threatens to sue for poisoning him, point out that he only took birth control and if anyone should be getting legal, it's the author.