I assume your friends are DVC members or at least staying on a member's points at VWL, but they have 1 more person in their family than a studio will accommodate. So I don't think they can pay for the
DDP (assuming they're paying for the DDP themselves) for the "extra" family member on their reservation. He will have to be on your package/room reservation and you will have to buy him the same type of MYW that everyone else on your package is getting (and pay for his DDP too, but obviously your friends can just pay you back for his MYW and his DDP cost).
Now, like everyone else on your package, he won't have just his "own" credits on his KTTW card, he will have access to ALL of your family's credits. So you need to be comfortable with that. Especially if he will be eating meals separate from you at times. For instance if he eats with his family, there would need to be 2 checks at that table, one for him and one for the rest of his family, and he or his parents need to make sure there aren't too many credits mistakenly deducted from his (your) account. You will always get a receipt showing the number of credits deducted and remaining, and so you'll just need to keep a sharp eye on that to make sure he hasn't accidentally had too many credits deducted when he is not eating with your family or that he hasn't, you know, run your snack credit account down to zero by constantly buying mickey bars and sodas.
Your 2 families could get 2 studios at VWL under one DVC reservation (if your friends are members they might need to buy points from another member to make this happen, or even you as a nonmember could rent a sufficient number of points from one member - check the DVC rent/trade board here for more info) and then you could get the DDP for everyone at once, and none of you would need to pay Disney's rack rate for your park admissions; you could get them separately from a re-seller at a discount. You might save a fair chunk of money this way (especially if you were not planning on staying on a Friday or Sat. night to begin with, since the number of DVC points required for a weekend night is double what it is Sun.-Thurs. nights). But everyone in both families would have access to all of the DDP credits, so you would still need to be OK with that. And availability at VWL for December might be somewhat limited at this point anyway. But it could be worth a look.