I think you should have responded with humor - waited until Mom came back, and said:
“Hey, look! Daughter finished all her milk! I thought you said she HATED milk! But not only did she drink all of it, she just told me she wanted MORE! Here, I’ll get some. No, no, really - it’s no problem. See?”
The objective is to make her really uncomfortable without actually ratting her out. Let her know you’re on to her, but don’t make yourself the bad guy.
I like this one. Although…
this keeps slaying me. I think I’m over it, but then it gets me again. 
Muffin, you win some kind of imaginary prize.
How involved do you want to be with her daughter and how involved does she (both g/f and daughter) want you to be? I’ve been in a relationship where her children were asking me for permission and coming to me as a father figure, and was accepted as such by the family (including her). If there was something like the above I could talk to them a a parent/child level and also talk to her.
That may have been a opportunity to talk to her daughter about it, and establish your relationship with her daughter. If she really doesn’t like milk perhaps she could ahve expressed this to you and you could talk with her mother and maybe could have changed it, showing her that there is a way besides sneaking behind her mother’s back.
Seconded … or thirded. I would have phrased it as “You know I’m going TO HAVE to tell your mom you did that, right?” I would suggest the OP have a conversation with his girlfriend about this sort of stuff immediately. If a child feels like she can exercise some power over a situation then she is going to go for it, as in this test. The kid needs to know that you are backing up the mom, not her.
I don’t have kids and don’t even really know much about kids, but I think if I were in that situation I would have said something along the lines of, “Hey, I saw that. And you know what? I’m going to give you one freebie since we’re still getting to know each other. We’ll pretend it never happened this time. But I just have to let you know that if it happens again, I’m going to have to mention it to your mom. 'Kay?” And let it go at that. One glass of milk down the sink isn’t a tragedy. And this way the kid knows where she stands for next time without any ambiguity.
Thanks (as long as the prize is not broccoli).
I see you got a couple of agreements on this. I guess I’ll be the first to disagree. Strongly.
Maybe he likes this women for more than her census information. Maybe he really likes her for who she is and doesn’t consider her interchangeable.
I like this, too…it’s too soon to get into it, and a positive approach is best for all involved. Heck, this is waaaay down the list of things to be terribly concerned about, although you don’t want to be setting any bad precedents or anything…
That was an option I was also considering - I like that phrasing, too.
I’m dating a woman with a 6 year old son so I have had to deal with this sort of thing a few times. I have always considered that part of my “job” as mom’s boyfriend is to back her up - he understands that it’s still mom’s rules even if mom isn’t there at the time.
I think that Cat Whisperer got it right - I would have done something similar; since the daughter was obviously trying to sneak something by her mom, let the daughter know that mom’s about to find out and then mention it to mom. You can do this completely calmly and in a friendly tone but the daughter needs to know that you aren’t her partner in crime.
And it’s not “tattling”, you aren’t a little kid at the playground running to tell a teacher that Bobby said “peepee”.
I was a little nervous about dating a single parent but we’ve been together for a while now and we’ve got things worked out pretty well. I might cook him something different for dinner but he’s still going to eat. If I’m kid-sitting for an evening he’s still going to do his homework/brush his teeth/go to bed at the right time and so on. His mom and I have different styles about certain things r.e. her son but on all the important stuff I do it her way.
Communicate the same message as mom does, be nice to mother and daughter, be a good man around them and see how things progress with the relationship. Talk to mom about how you should handle situations like the milk thing.
I like her because we’ve had a relationship before, when were young. We were together for almost two years right after high school, then had no contact until about a month ago. It turned out we weren’t compatible back then, but things are much different now… magically different. We’re both at a point where we want the same things out of life… but I won’t get into all that. Funny how twenty years of growing up can change things for the better. 
That’s awesome.