Psychology isn't a facilitating subject for either career, so is the outlier.
If she goes down the medical route or allied careers nearly all want 2 sciences. If she doesn't want to take 2 science A levels, then she probably won't like the degree content.
Perhaps look at the modules in each year of a medicine degree at the universities. The reason Chemistry is often required is there is a high content of pharmacology.
Also when applying for a highly competitive medicine undergraduate place you are up against the candidates with Chemistry and Biology, so you are more dependent on your UCAT score. Once you get there you will have to work harder to make up some lack of knowledge compared to others in what is already a highly demanding degree.
With law at the top universities you need A levels you are capable of getting an A star in ideally.
What is her likely overall 8/9 profile for GCSEs?
Science A levels you really need high 8 to 9 for A at A level and taking one science only does tend to put you at a slight disadvantage.
2023 results for subject A/A stars:
Biology 9% A star, A 18%
Psychology 5% A star,13% A
History 5% A star, 19% A
Essay subjects tend to be harder to get A star in -which does not mean sciences are easier at all, just there is a definite right and wrong and accumulation of marks -so psychology isn't a safer bet of a top grade.