You mention strong performance of the investments, but then your questions are about dividends. Just to make sure you've got the full picture for UK taxes, HMRC taxes both dividends and capital gains (similarly to the US, but differences in the details):
HMRC taxes dividends above the dividend allowance (£2,000 for 2022/23, £1,000 for 2023/24, and £500 for 2024/25 onwards). There's no differentiation for qualified dividends, all dividends are treated the same (and no complication about reporting funds - still a dividend either way). So you'd need to report all dividends from 06Apr22 to 05Apr23 on the UK Self Asessment, converting into GBP. I use HMRC's monthly exchange rates, as a reasonable balance between looking up every individual day vs an annual average (and given the USD/GBP fluctuations last year, that annual average would hide a lot of movement).
https://www.gov.uk/government/publications/hmrc-exchange-rates-for-2022-monthlyHMRC taxes capital gains above the capital gains allowance (£12,300 for 2022/23, £6,000 for 2023/24, and £3,000 for 2024/25 onwards). There's no differentiation for long or short term gains, just gains. But for funds that are NOT HMRC reporting, these are taxed at the income rate, not as capital gains. You don't mention what the investments are - if they're individual stocks, you're fine. If they're mutual funds/ETFs, need to do some digging. The official list of reporting funds is here:
https://www.gov.uk/government/publications/offshore-funds-list-of-reporting-funds. A more human-readable list of common US index funds that happen to be HMRC reporting is here, but if the fund isn't on this list, it isn't a guarantee it's not reporting, particularly if it's an active fund:
https://www.bogleheads.org/wiki/US_domiciled_ETFs_that_are_UK_HMRC_reporting_fundsJust like dividends, you'd report all capital gains during the UK tax year, converted into GBP.
Because of the different tax years, US 1099 forms are pretty useless for the UK. You need to do your own calculations for the UK tax year, but since there's no differentiation of qualified dividends or long vs short capital gains, it can be pretty straightforward. Where it gets complicated is calculating capital gains cost basis with multiple purchases and multiple partial sales. 1-to-1, many-to-1, or 1-to-many are all pretty easy; many-to-many is doable but much less fun. HMRC doesn't give you options like FIFO, SpecID, etc. - only one way to do it:
https://www.gov.uk/government/publications/shares-and-capital-gains-tax-hs284-self-assessment-helpsheet/hs284-shares-and-capital-gains-tax-2019Note that all the above applies only in a taxable account. If your daughter was investing within a tax wrapper recognised by HMRC (IRA, 401k, etc.), your life becomes much simpler