I agree with both Dexter and Sleeps. Rules are rules. Your friend probably would have preferred to you as being the best man.

As for not being allowed to take communion, there are 'steps' in Catholism. Even if you were baptised you still won't be able to take communion until you've taken up the 'study' and gone through the 'passages' so to speak. It's like the graduating driver's license. Here a new driver has to take a written exam, then practice, then the first road test, practice (can drive but with restrictions), then wait another year to take a second road test and then after passing the second test will the the person become fully licensed.

I hope that by not having you as the best man will not ruin the friendship. Perhaps you can play a role in other ways, like being an usher or something.