Hi L.,
I think if you are close enough to the girl's mother, you should talk to her. I wouldn't start off by asking her why she discussed matters with her daughter that you two had talked about. I would simply let her know that your statement was not meant to cause any confusion and you apologize if it was taken the wrong way. Let her know that you were not accusing her daughter or your daughter of doing anything underhanded because if that was the case your daughter would not have made plans for the boys to pick them up at your house. And in the future you will know not to say anything to the mother that you don't want to get back to the girls.
If you don't feel comfortable talking to the mother, talk to your daughter. Explain to her that you didn't mean it the way that it was told to her or her friend. And you apologize for any confusion. Sometimes as parents we do have to lead by example. You apologizing to your daughter is definitely not a sign of weakness. It's simply letting her know that yes, you said what was said but you did not mean it that way.
The last thing you want is for your daughter to listen and believe any negative things that they may tell her in the future about something that you have said.
Now if she decides she wants to continue to be upset with you. Just know that you have done your part. And remember this too shall pass.