Tax free trading allowance

First of all, it is perfectly legitimate for someone operating a lettings business to pay people to work for them and, in all probability if your daughter claimed Fees of £1000 a year it will be accepted by HMRC without question.
However if they did decide to make an enquiry I see a number of potential problems. In this case, you are the letter's father and HMRC are very wary of family relationships between payer and payee.
First problem: Are you an employee of your daughter or an independent trader? Unless you have other, non family, clients paying you for a similar service I would say that the odds are you will be her employee. If so, no trading allowance for you and she may well have to operate PAYE on your wages.
Second problem: How much time do you spend on this work and how much would be reasonable for her to pay you? If you are an employee that has to be at least minimum wage. I see from your earlier posts that you have BTL properties of your own so you presumably have some expertise in this field but are not professionally qualified so payments at a rate comparable to estate agents couldn't be justified.
Third problem: As you, yourself have BTLs, is your daughter's business genuinely her own or has it been funded by the bank of mum and dad?
Are the profits hers on paper but yours in reality?

You'd have to register with HMRC for money laundering regulations and maybe also for data protection regulations - there are rules/regulations which apply to book-keepers and accounting service providers.

If you have unused personal tax allowances, then as suggested above, may be able to achieve the same result by merely being an employee - yes, there'd be PAYE/RTI requirements, but you'd avoid the money laundering and data protection issues.