Despite what another poster said, Bunac are still relevant.
@pilates there is a way of doing it if she is currently studying at university at degree level (although not appropriate to her situation, this also covers those who graduated up to 24 months beforehand as well).
Government Authorised Exchange visa (Temporary Work)
https://www.gov.uk/government-authorised-exchange
There are a few things that she needs to arrange but these can be arranged by companies that regularly do this sort of thing. One example is a company called BUNAC (British Universities North America Club).
They were originally set up to help British students get jobs as camp counsellors etc at US summer camps (eg arranging job offers and visas etc - I got a summer job in the US through them back in 1985)
The link above is from the official gov.uk website explaining all the details.
Basically, BUNAC act as sponsor and help people find an employer. They call it their "Intern in Britain program"
https://bunac.org/internships-abroad/intern-in-britain
and explain about the visa here:
What’s the Temporary Worker GAE visa?
Formerly known as the Tier 5 visa, the Temporary work GAE (Government Authorised Exchange) visa is the UK internship visa for international students and graduates. It allows you to come to the UK for a temporary period to do short-term work experience.
In order to apply, you must have a sponsor (that’s us), meet the eligibility criteria (that’s you), and have a paid or unpaid internship placement offer from a UK company (that’s your future UK boss... try not to mess it up at the office Christmas party).
and details of how they help with finding an internship are here:
https://bunac.org/internships-abroad/intern-in-britain/finding-an-internship
Although, of course, they do charge for this. If she can find an internship by herself then they charge US$1,549 for their sponsorship and other services.
If you use their intern finding service (it's basically just a job board, like Monster Jobs or Total Jobs but focussing exclusively on internships) then they charge US$2,179.
To be totally frank though, I have no idea if those fees are worth it or a rip off.
In addition to their fees she would also need to pay £298 for the visa and, if she is coming here for more than six months, then she will also need to pay the
Immigration Health Surcharge of £1,035 (this is basically health insurance, so that she will be fully covered by the NHS and there won't be any co-pays etc. Although dental treatment is excluded and she will need to pay the standard prescription charge of £9.90 - although some prescriptions, such as contraceptives, are free of charge).