Hello, Helscam1. I'm sure this must have come as a surprise to you.
You have received some very good information so far. If you are not aware, there is a publication on the IRS site which will help. Search for Publication 54.
Unfortunately, US Citizenship comes with obligations no matter where in the world the citizen lives. It's fairly unique in what it requires. Those obligations remain in force forever as long as the person is a US citizen.
As has been noted, your daughter's future financial planning will be impacted by her US citizenship. Quite simply, her US responsibilities will determine her future actions on investments, possibly pensions, and even simple bank accounts. It should also be noted if she ever wishes to take advantage of any UK benefits, these might also need to be reported. A profit from the sale of a UK property, if sufficient, may also be taxable by the US even though it is tax free in the UK. If she has a non-US partner, any joint accounts might require reporting and the partner should be made aware of this.
Something that is seldom talked about, but due to your daughters age, is registering for the Draft. I know nothing about this, so I'll leave it to others to point out my possible error on this. I only mention it since your daughter may wish to pursue advanced education in the States. Financial assistance may be restricted if the person has not registered for the Draft.
For those with a long US Residence before moving abroad giving up US citizenship requires a great deal of consideration. In your daughters case, it will come down to a consideration of maintaining an option to live in the States at some time in the future. A very subjective and personal decision. But again, as long as she is a US Citizen, she should attempt to abide by the rules going forward.
You have mentioned she has a US passport. Read the fine print in the passport. It explains that as a US citizen she must file tax and information returns.