If you're looking for a ballroom Slow Waltz at the standard tempo of 28-30 mpm (84-90 bpm), I'm afraid you won't have much luck. The Beatles wrote many songs, or parts thereof, in a Waltz-able time signature of 3/4, 6/8, or 12/8. The problem is, the're all of a Viennese tempo, which is nearly double. You can slow them down slightly, but not enough to get them down to Slow Waltz speed -- at least, not without making them sound ridiculous.
So if your daughter really wants a Beatles song, she'll have to learn some form of fast Waltz. I strongly recommend you ask your dance teacher for a combination of "Hesitation Waltz" and "Vintage Waltz", both of which will be feasible with only a few weeks' preparation time. Do not attempt a standard ballroom Viennese Waltz. The difficulty level is too high and the time too short.
Here is a list of Beatles songs in a Walte-able time signature:
Be aware that most of these are not appropriate from the standpoint of lyrics, especially Norwegian Wood, She's Leaving Home, and Hide Your Love Away.
Now if you have a live band, you do have one more option: time signature conversion. Almost any song can be converted from 4/4 into 3/4 or 6/8 if you have a live group of musicians who are capable. Some songs lend themselves better to this conversion than others, so discuss the possibilities with the band leader well in advance. One example of a song well known to work in both time signatures is Fly me To The Moon. Another example of a brilliant conversion is Michael Jackson's Billie Jean, which was done in 3/4 by Chris Cornell.
Even the Beatles themselves have a song they played both ways: I'll Be Back. Here's the original 4/4 version, and here's the 6/8 conversion. Again, this has the same problem as all of the songs on the list above, which is that the conversion is to a 6/8 time signature, played at about Viennese tempo. But there are probably many Beatles songs that could be converted to a slow Waltz in 3/4 at the appropriate tempo, assuming you have a live band.
Good luck finding the right music, and enjoy the moment!
Regards, Jonathan Atkinson www.ballroomdancers.com