Well, he should ask his GF to have a household bedtime. I can't imagine it's good for them, either. And if not, even if it's harder for her to go to bed first, she won't be the first kid to be sent to bed earlier. I used to go "but Mom, there are still kids playing!" and she said, "No my kid. It's time for bed."
You can also ask him to wake her by 9 or something and then have her have "quiet time" (nap) around 3 for an hour.
And in further discussions with him, emphasize that it is not working well FOR HER to be up that late, as it is affecting her entire day on Monday, her ability to do well in school and her lessons.
If you get along with the GF, could you appeal to her? After you and ex agree that there's a problem, the next time it comes up mention that Emmy needs to sleep earlier and you hope she will support ex in getting Emmy to bed before midnight. It can J. be a statement. My DD is a night owl, too, but I do NOT let her stay up til 12. Her kids probably do need sleep but she's not enforcing a bedtime.
I totally get that you can't control his home. My sks were routinely dropped off late on Sunday nights and Mondays were no fun. Sometimes you J. do what you can. I would also consider moving the piano lesson to another night. If Mondays J. don't work for her, then don't set her up for failure.