I assume your daughter is 18 or older?
If she's between 12-17, she will qualify as fully vaccinated in Spain according to the UK Government guidace. She therefore would not need a test.
https://www.gov.uk/foreign-travel-ad...y-requirements
If you are aged 12 to 17 and have not received the booster, but have received two doses of a two-dose vaccine or one dose of a single-dose vaccine approved for use by Spain, then at least 14 days must have passed since your last dose, to qualify as fully vaccinated. You continue to qualify as fully vaccinated more than 270 days after your last dose and do not need to show proof of having received a booster to travel
https://travelsafe.spain.info/en/
This says the same. I wasn't able to copy the text into the post.