Going just to study English, for more than a month or two, is going to be tough - some would say boring. If she can get by then there are all sorts of opportunities to develop her language skills by using them in real-life. (As long as she avoids the trap of hanging out with Mandarin-speakers all the time, of course.)
What age is she, and what work experience does she have? Her options are determined by her background.
There are, for instance, foundation courses available at some (good) UK universities which teach English together with specialities such as art and design.
Or she can do something vocational, such as a course in hotel and tourism which includes PAID work placement and offers a flexible work permit at the end of it.
Actually, some UK universities are now able to guarantee work permits for overseas students, valid for two years after graduation. I presume she wants to improve her English for professional reasons, so why shouldn’t she gain some international exerience too? Mandarin-speakers are quite employable these days.
And don’t worry about the accent rubbish. Unless she’s extremely advanced already she’s going to come back with a Taiwanese accent.