It is no longer compulsory to study a MFL in KS4.
(Interestingly though, it will soon be compulsory to study one at primary school!)
I would warn against dropping German for the reason suggested by herman...if the DC is considering an academic course at a university. If you can get a D, you can get a C with some support. Try to get some 1-1 tuition, and as already suggested, podcasts, online news in German, even newspapers and magazines. It's always worth speaking to the school too, and explaining the trouble - it might be that the teacher could help.
are links to online German youth magazines. There are loads more: if you search deutsche Jugendmagazin
you should be able to find them with a basic knowledge of the language.
Tell her to stick with it: it's a great language!
And I would love to help your DS, Moved, but you live too far away. Move again?!