There are two conflicting laws
- US citizens must use a US passport to enter the US
- A US citizen cannot be denied entry
The going legal opinion seems to be that #2 will trump #1, so if your daughter shows up at the border with sufficient proof of citizenship, chances are she will eventually be admitted. See for example https://law.stackexchange.com/questions/16654/can-us-citizens-be-denied-entry-to-the-us
However, getting to the border itself will be difficult. No airline will allow her to board with an expired US passport.
The solution is easy enough: she should get a US passport.
If you are in a hurry, do this: https://uk.usembassy.gov/u-s-citizen-services/u-s-passports/emergency-passports/