Do you have any other commitments currently? What school level of maths/physics do you have?
My advice for them would be to take it seriously, ask lots of questions to whoever can answer them, if you can do anything extra at all to help learn stuff then do it (I only had the opportunity once or twice, but going and seeing some mid-maintenance aircraft with cowlings off etc and speaking to engineers helped me with a bit or two.) Use question banks to brush up but ONLY TO BRUSH UP. I don't know the current state of the exams but when I did them, the guys and girls who tried their best to learn the material and then turned to the banks in the run up to the exams did far better than those who rushed through the material and did the same questions over and over.
With regard to the first two questions I asked I will give my personal experience.
I had decent GCSE grades in Maths and Physics and 'OK' A level grades but nothing great. I didn't find any part of the maths or physics challenging. The physics is mostly about understanding some basic principles and if you can get your head around some basic trigonometry and formula manipulation I think that covers most of the maths.
I was working a mix of part-full time when I was distance learning (0 hour contract, sometimes 35+ hours a week others <16) and I managed the exams from the day the books arrived to the day I gave in my last exam in 7 months with a 1 month break in between the 2 modules. If you have no commitments then 6 months or less should certainly be possible, if you are working full time I would say it would be very tough.