[..]
She immediately contacted her manager (who was at home at the time), who in turn has contacted the area manager.
They have pretty much said that they aren't going to act on it because
- it will be her word against his
- a drug test wouldn't prove he was doing it on shift (WTF), he could have done it in his own time, he might have done it in his own time and the smell was in his car then when he sat in it on his break it then transferred to his clothes without him actually smoking it.
- he might turn it around as her being racist (again WTF)
Arse-covering, corporate style. But they're correct in both of the things you state "WTF?" about. The drug test for cannabis doesn't even test for cannabis. It tests for metabolites and is far too crude a test to prove time of use. It's a crap test, frankly. It should be obvious that playing the race card is a real thing nowadays and is a genuine risk for any business. Reality is irrelevant when guilt is presumed because people are deemed to be the wrong sex, the wrong "race" or whatever irrational prejudice is fashionable at the time.
The company's sole concern will be the company, so people at decision-making levels will be after the route of least damaging exposure. Doing anything about it will definitely result in negative exposure,
extremely negative exposure if the person plays the race card. Doing nothing about it might result in no exposure and probably won't result in extremely bad exposure because the drug involved is cannabis, which currently has a good public reputation.
If she wants to take a risk by going against the company, she could push the issue within the company and threaten to escalate the issue by going public if the company doesn't meet her demands. That probably won't go well for her. The care industry is not exactly known for being considerate of its employees. She could try an "anonymous" (*) call to get the police to do something and then lie to her employers if they ask her about it. If he really is smoking cannabis at work, he'll probably be influenced enough to impair his driving and that would give her plausible deniability if the police did pull him over and test him. That would probably be her safest way if she's a good enough liar.
* It probably won't really be anonymous nowadays, of course, but it shouldn't be accessible to her employer.