Oh my goodness Kruck - that seems like a big ask. Does your friend mean you would be looking after him full time as well, as they are working? As I'm not sure where that would leave you in terms of technically being childcare, and if you'd be paid then you might need to be registered etc. I think. Or is it just to do particular subjects with him?
I guess in a way at 11 he would be partly self sufficient, so wouldn't be the same as adding a younger child to the mix. You'd presumably be able to set him some work to do and let him get on with it while you do other stuff. But then at that age, particularly if he is struggling with behavioural issues (am making an assumption there as you say he is near expulsion), it could be very difficult, and I guess you'd need to think about whether he'd be coming with you to toddler groups and that sort of thing.
In terms of the actual education side, it's a lot easier than it sounds if you're reasonably confident about it. Lots of people say oh I could never home ed, because I could never be a teacher - but it's totally different, there's no way I could teach a whole class of 30, but working with individuals or just a few kids at a time requires different organisation and skills. It's quicker as well, so if you were HEing one teen you wouldn't need to be spending 9-3 at the kitchen table IYSWIM. You don't need to stick to a curriculum (in England anyway, not sure about anywhere else), so there's no real need to do every subject, or there might be more unusual ones he's interested in - although doing maths/English/science would be advisable in case of future plans, generally. I'm assuming that even if you take over most of the actual home ed stuff, the legal requirement (a child must be provided with an education suitable to their age and ability, at school or otherwise - the home ed being the 'otherwise') would still be on his parents, so any LEA involvement etc would still be through them - basically they would still be the ones who have the responsibility to make sure they/LEA are happy with what you're doing. I think.
It's hard to say more without knowing what your friend is asking you to do exactly, but if you want to chat more about it then that's cool, by PM if that's better. :)
My maths class went pretty well today, it was quieter due to a couple of absentees but it was pretty successful and although I slightly diverted from the lesson plan they certainly learned a lot from it. :)
and 
on the bar. 
back on today the last couple of days have been rubbish no motivation and the house is starting to slip again 
so got the kids settled watching a film and gonna try and crack on with the housework! Hopefully get everything sorted including the 
shoot me now !
)