Neither I nor DS went to Cambridge, but we did study economics.
College choice probably does not matter that much, but economics is a broad subject and it may be worth looking at the academic interests of the fellows (if that is what they are called) within a college, and see if they align with her own emerging interests.
As well as the obvious opportunity to be tutored by someone whose work you find interesting, you may strike lucky. A friend's daughter chose an Oxford college for English because she was interested in the work of one academic, and she was interviewed by her and so more able to demonstrate her informed enthusiasm for the subject. She was offered a place.
DS found his Cambridge economics interview very formulaic with set questions including a number of maths questions, that, for some reason, he stumbled over. Perhaps they were more open ended questions than he had realised, and he did not offer enough. He did not get a place. Back then Trinity used to have a reputation for being very mathematical, and when DS was applying used to require applicants to take STEP. I don't know if this is still the case.
Off topic, but it is worth being aware that economics generally is quite competitive. The more mathematical courses: Cambridge, UCL, Warwick and LSE are all oversubscribed and will reject a good number of qualified applicants. There is a decision to be made about whether to apply to all four and hope one offers a place (and it is perfectly possible to be offered a place at Cambridge, but be rejected by one of the others) with a gap year and reapplication as a fallback, or look at a wider range of Universities first time round.
And perhaps worth considering whether she wants a more mathematical course or something broader. Ie is it Cambridge rather than Oxford, as opposed to Economics as opposed to E&M or PPE.