Greg-
I believe that you need to go to the bride and do the mea culpa. Find out what it will take to mollify her beyond admission of a mistake and asking for forgiveness.
THEN, you need to deal with the bartender that you hired.
You might want to get some input from the coordinator before speaking with the bride.
I have no idea about bartenders and tips, other than in bars. Is it routine that they receive tips?
And what about the groom and both parental units. Are they up in arms, too. Or is it just the bride that is overly cranky?
Some advice and some things to consider.
JayR