It is obligatory to pay the khums from your own money, not from the money of other people. If the money is mixed with the money of others, it does not effect this obligation, so it is better for each person to keep account of his share.
If he broke the intention of fasting and decided to break the fast, even if he did not actually break it, his fast is void and he has to make it up. However, he should still refrain from the things that break the fast for the rest of that day.
He does not have to pay kaffarah.