YAY for the date!
Yeah, I suggest be flirty but honest about your friends too - say sort of what you told us, maybe with a little humor like Tiercel suggested (maybe don't mention the hawk thing though, just in case

) and then ask if her friend would still be interested in going out, is it okay to get someone 'just for fun'/to even the numbers (who knows what can happen anyway? but the girl shouldn't get her hopes up), or if it can be just the two of you? Or if she has a better suggestion?
Sometimes it's also good to not date every day, maybe something to keep in mind for later dates? so things remain more interesting longer? of course go through what you agreed upon already, just tell her about the friend situation..
You are not a dating agency, so just keep things real. Some people do meet through friends, and she just asked - possibly having no idea she'd cause any distress..

Worst situation, the girl's friend could get a guy she met online to come with too?