Are you being willfully ignorant, or did you just not read any of the OPs posts?
Did you not read my post? I didn't say anything about holidays. Although based on my experience, I don't know of any students who don't at least come home for a few weeks just to visit their parents, even the ones who live with their partners.
At 18, there is a pretty good chance she will not still be in a relationship with her boyfriend by the time she graduates.
The OP says nothing about how much money the DM gives her DD, only that she gives some money to the BF's DM for board. The DD still needs to buy clothes, pay for travel, buy lunches and snacks, books, equipment, pay for a phone, have a social life, maybe pay for field trips.
Why shouldn't her DF pay 50% of the loan top up/actual costs, as I suggested? If it is less than the CM he is paying, then good for him. Although the loan and parental contribution are only supposed to cover the academic year. Given that there are very few holiday type jobs at the moment, their DD might need financial support in the summer. The government expects parents to financially support their children through university, why shouldn't he share those basic living costs?
I am guessing that you are going to riposte that the DD has reduced costs at the moment because of lockdown and because she is living with her BF's family. It might be fair to agree a temporary reduction to take into account she has no travel costs. Personally, when it comes to saving money by living with her BF, my belief is that parents should still pay the full means tested top up amount (unless they really can't afford to). If my DC choose to live frugally, they should be the ones to benefit by reducing their loan debt as they are the ones making the sacrifice.