I read History at Cambridge, but A-levels were unfortunately 24 years ago, so do take with a pinch of salt. I took Hist, Eng lit and G&P myself.
If she wants to study History, I would urge her to think of each of her choices in that light as well as on its own merits.
I would defend her interest in Government and Politics - it's an excellent course, or was, and in my experience was not at all a soft subject. Is she interested in political history? What evidence can she offer of this? Does she like reading political memoirs/diaries/biographies? Is she a member of a political party? Has she visited the HofC? Has she looked into internships, visits or something like this to the European Parliament? is she a regular listener to the news, World at One, anything like that? Is she Roundhead or Cavalier? If she had to debate with either Gladstone or Disraeli, which one and why? 
I am much more dubious about Theatre Studies. I hope I'm not automatically snobbish, but unless she has a passionate interest in theatrical history and can show evidence of this, I can't see the relevance of this subject to her university course. Could she not join a youth theatre in her own time instead? Or stage revivals of historical plays?
You are spot on about a language. I found my options at Cambridge severely limited because I had such poor language skills and therefore my ability to handle original sources was restricted. I felt unable to take anything from the medieval period backwards as I had such bad Latin, and couldn't do much modern European history because I had no German or Russian; 17th to 19th century very difficult due to poor French. Now, this may partly have been due to my inability to ask for help at Cambridge, but I think I was realistic in what I eventually studied (early modern British and 19th C American). It would have been nice to have more choice, really.
It's great that she wants to study physics. I personally would never stop anybody who wanted to study a science. I hope she continues with this. Some reading around the history of science before her interview may help. History of Science and Philosophy is a course that is a well-known seducer of scientists at Cambridge - not sure about Oxford.