While the sentence does talk about "someone's daughter", I'm not too sure about the details. I feel like the sentence made it vague, but I'm willing to do a revision if you think it needs one.
The sentence in question is
...知り合いの
ご令嬢に
マリーさんなんて
娘いないはずだ
All I can really think is that the true name of that person is being hidden, or it's also Marie as well. Can't be so sure until we reach Volume 5
Either way if we do learn of the truth eventually, I'll be sure to edit it in here