The great thing about PPE is that there are no necessary A Levels. The standard (and almost universal) offer for any degree at Oxford is 3 A grades, and as long as you have those it doesn't matter what they're in (apart from General Studies and Critical Thinking, of course). Any other qualifications will not have any bearing on whether you will be offered a place or not. What will matter is the grades you achieve at AS Level (4 As at AS level is a good benchmark to aim for, but 3 would be fine as well), and the reference from whichever teacher is responsible for your application form. Any other internal exams or previous results are very unlikely to affect your chances.
Where Maths is described as "useful", it is because there are some maths skills at A Level (mainly calculus and the use of indices) which will be needed for the economics part of the course. Anyone who doesn't have those skills i.e. didn't do Maths A Level will be required to attend maths lectures to teach them the skills in order to be on a level footing with those who do. What is helpful though is that anyone who DOES have Maths A Level, but is perhaps not very confident in their Maths ability or could do with a little extra help is welcome to attend the lectures too. In this way your performance in Maths is not a major deciding factor when it comes to offers being made, because you can catch up once you get there. As long as your friend is performing well enough in other subjects to make the grades Oxford would want, his maths specifically should only be a minor factor, and, again, probably won't have that much of an effect on his application.
Hope this helps!