Canceling the ticket will likely lose the DBC voucher entirely.

Sorry you got the run around, but I think the only complaint you really have is the rude CSR -- even when your friend travels, standby on alternate routing is not usually allowed unless there are problems, e.g. weather or mechnical issues (there are exceptions to this, and you should still try but its less likely than just catching an earlier flight).

Send an email or letter in about the CSR, hope you got his name, and if you don't like their canned response follow up with a reply. G'Luck.