No-one so far has mentioned the other friends. They are part of the affair, since they must have put their names, along with yours and Bob's on the present, one would assume. One would also assume they are/were Bob's friends too.
If they are not aware of the situation, they should be. This then could be an answer to helping him be convinced. By the time he's heard from the rest of the group how unsatisfactory his behaviour is, he may come to realise which friends he may be losing, if not just giving a bad impression to.
Maybe something along the lines of ' Did you realise we've all had to chip in more for that present with your name on too, because it's not fair on Axel2D to cover both your share and his/hers.'
A desperate final idea may be to explain to the recipient of the gift that a mistake was made when Bob's name was put on the gift. Desperate as I say!