If she’s under the age of 19 and is in further education, she falls into the category of students who are eligible to claim HB.
It would seem to be a bit of a peculiar result however to treat her as part of his household as a dependant child on his share of the rent and then determine a seperate claim for her on her own share of the rent.
Could you not assess the father on the full rent on the basis that they are jointly and severally liable (and therefore each liable for the full rent in its entirety)? Possibly you might justify this under Reg 8(1)(c)(ii) – if he has been paying the rent in respect of both of them.
If she has been paying her own share then you might want to consider inviting her to claim in her own right.