Unless there is an exemption that I am unaware of (likely) then unless you friend has an approved isolation facility then you cannot do a day visit. The movement need to be accompanied by the movement licence, and it will trigger a six day standstill after which you can bring her home again. The movement home also requires a movement licence and will trigger another standstill.
I don't know if AH/DEFRA would ever check, but I am wary of not reporting movements to the billy as if asked about it, how do you explain the sudden emergence of kids in 5 months time? AI leaves you with paperwork, a sly visit to the billy doesn't
This is why my girls go for a whole month, otherwise with 4 of them I would be on standstill for ever