I can see why you're unsettled. A five year old could certainly see a scene in a movie of an open mouthed kiss. Kids are usually grossed out by such an image because it seems disgusting to them. It is also entirely possible that a five year old could ask her parents about it-- particularly if she's disgusted-- and be told that grown-ups kiss that way, and "it is beautiful." That same five year old could then want to try such a kiss with her Barbies or her friends, because she's curious, or even because she's trying to work out her own feelings of disgust. If that were all that had happened, this would not be too concerning.
However, I can't imagine a parent telling a five year old that "that's how princesses kiss." I can't imagine an image of a princess that would include such a kiss. So the comment by this five year old that "that's how princesses kiss" is a red flag that someone has been giving her inappropriate information. What's more, it sounds like information that's designed to encourage this girl to kiss open-mouthed, given this five year old's interest in princesses.
This alone would concern me enough to have a conversation with the mom about it.
The plot thickens, of course, when we hear the five year old talk about her dad. Most five year olds would not describe their parents as "cute" or "handsome," but by itself it's just odd, not alarming. However, I can't imagine a five year old saying "He's a good lover." Sure, this could be kid-speak for "he's loving" but most five year olds wouldn't even say that. They would say how THEY feel: "I love him!" To say "He's a good lover" sounds like she is repeating something she's heard from an adult. Now, it is entirely possible that this girl has heard her mom on the phone saying "And he's such a good lover!" so she is repeating that. But the fact that she says that's why SHE likes him suggests inappropriate ideas about the parent-child relationship.
Does this mean that this five year old is being abused? Clearly, we have no idea, and these are not allegations to make lightly. But are these red flags? Absolutely. Are you being paranoid? No, you're paying attention to those little red flags. Could this just be popular culture? I don't think so. Barbies don't kiss open-mouthed in popular culture, and girls love their daddies because they're daddies, not good lovers. Young children don't conclude that their daddies are good lovers even if they hear such a comment in a movie.
Most mothers whose children are abused by their partners don't know it's happening, because it is just so hard for us to believe it. However, since about 20% of adult women in the US recall being sexually abused as children, and at least 30% of abusers are family members (some estimates are double that), we know that many, many children are being sexually abused by their own parents. That means that each of us may well know a child who is being abused, as unthinkable as that is.
Whether you can or should approach the mother of the child is for you to decide. Clearly, if the child is in danger, you want to say something, but you have to do it with a light touch. You might simply tell the mom about the kiss your daughter gave you, and her daughter's comment about how princesses kiss. She will probably make light of it or say your daughter misunderstood, but at least that plants a seed so she can be more aware as she observes what's happening in her home.
In the meantime, I wonder what your daughter has been exposed to. I recommend that you get your hands on the book My Body Belongs to Me by Jill Starishevsky. I believe this book should be on the shelf of every young child, in the same way we have books about emotions, or potty-training. It deftly presents the concept that the child's body belongs to him or her, and there's nothing inappropriate, scary or suggestive about it. However, if a child IS touched inappropriately, this book can help the child to talk about it. In fact, if you happen to be reading stories to your daughter and her friend at some point, this would be a terrific one to include, for the benefit of the other child.
Children work out their feelings using play, so if this five year old has some issues based on her experiences, they might well come up in the context of her play. You don't necessarily have to stop the playdates, but I would strongly urge you to keep them at your house, and to keep a watchful eye.