I agree that dropping maths will not be an issue for university admissions. As has been said, universities make sure to cover the essential maths for the course in their own modules- especially the statistics side of things as this is not a compulsory module at A-level.
As has been said, having poor results across the board because she was struggling with maths will make it harder to get a university place. Universities will sometimes offer slightly lower grades to those with more science subjects, but this will only be, say AAB instead of AAA and not all universities do this.
However, she also needs to be aware that dropping an AS could limit her university options, as some universities will ask for a 4th AS in their offers.
I think it comes down to how badly she is struggling in maths and chemistry. I think it is worth talking to her teachers about what they think she will get at AS in these subjects. If for both they think she will get a C/D or lower, then it will be very hard work to get both up to an A/B. If she is on track to fail these subjects, she really needs to think about dropping one. If she is on track for an A/B in classics, she is much better carrying on with this. If she is looking at getting less than BBC overall for her AS levels, she will struggle with getting a place at university.
Maths A-level is hard for people who don't "get" it, and there will be another step up next year.
If she is really struggling with chemistry, is she sure that a biochemistry degree is for her?
I do think a tutor might help if you think she has gaps in her knowledge.