would Bob have to make note of the value of btc every time he receives some?
If Alice is "helping out" Bob, is this a loan, or gift, or income to Bob?
If income or loan, he would want to record each payment, qty and date. The market value can be determined from the exchange rate under a few different ways.
So then let's say it was income, and Bob holds onto the coins the entire year. He has income from receiving the coins at the value when they were received. Hey may have unrealized capital gains at the end of the year but until he cashes them out (or spends them) those need not be reported.
If instead this was a loan, and bob started repaying, all interest payments would be converted to USDs and he could expense that as interest expense. But other than that it doesn't matter if the value goes up or down on the value of something that was borrowed.
Related:
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http://en.bitcoin.it/wiki/Tax_compliance