I don't think you can decide the right course of action until your DD has her A Level results.
To make a post qualification application,they need to be stellar. So on paper, she needs to be as good as, or better than she was this year. It also depends on what she is doing in the year off. If she is doing something related to the subject and immersed in it, she improves her chances. Mid however, she is doing something unrelated, she is probably in a worse position, because on a day to day basis, won't be interacting with the subject (as she was at school) and will be further removed from it than when at school. Of course, if her love of the subject is such that despite doing something else for her gap year, she spends all of her free time immersed in languages through choice and love of them, she may well still have a good chance.
I think a post qualification applicant has to LOVE the subject and not just want Oxbridge. I know a girl who has recently re-applied for a humanities subject. Her Sisters both went to Oxbridge and her parents. She really wanted to go too and seemed to be a near miss first time round. Now she is on a year out working with a big bank. She reapplied with 2 A* and 2 A (2 lots of maths). When I met her to discuss her application and do an interview, it was obvious she had not really done much reading since A Level. When asked about it,she explained that she was working full time in the bank and didn't have time. I gently pointed out that those who love their subject always find time, because it is a delight not a chore to read. She realised then that her love was for Oxbridge not the humanities subject. She had her interview and was turned down again. She started from a weaker starting point than first time round, because whilst good, her grades were nothing out of the the ordinary for Oxbridge candidates, and she had not really connected with the subject for 6 months.
So consider, are the grades good enough, and is there a love of the subject that is genuine enough to keep a teenager no longer in school, immersed in it of their own free will? If so, another go is probably worth it.