Well, for one if she has not filed a US federal tax return for over a decade then I seriously doubt that she would have any type of tax debt that would prevent her from renewing her passport.
The IRS has a website with information concerning the issue of tax debt and passport renewal here - https://www.irs.gov/businesses/small-businesses-self-employed/revocation-or-denial-of-passport-in-ca...
Just to check she can go to this IRS website for a tax account to see if she has a tax debt - https://www.irs.gov/payments/view-your-tax-account
If her income is foreign sourced, then as you mentioned she is eligible for the Foreign Earned Income Exclusion reported on a Form 2555 which is included with the personal tax return, Form 1040. The maximum exclusion amount for tax year 2018 is $103,900.
So if her foreign income falls below the maximum exclusion amount, the tax return that would be filed would have no taxable income being reported, therefor no taxes owed and no tax refund.
The exclusion amount changes for each tax year. For example the Foreign Earned Income maximum exclusion for tax year 2006 was $82,400.
TurboTax only supports the current tax year and the prior three tax years. So that would be tax year 2019 thru 2016.
You can get free federal tax forms, schedules and instructions from this IRS website - https://apps.irs.gov/app/picklist/list/priorFormPublication.html
IRS Publication 54, Tax Guide for U.S. Citizens and Resident Aliens Abroad may also be useful - https://www.irs.gov/pub/irs-pdf/p54.pdf
Unable to recommend any tax preparation firm in the UK that prepares US federal tax returns. However, with the number of US citizens living and working in the UK it should be fairly easy to find a local firm using an internet search.
