@lurchermummy MTPW- I know London well. I am a Londoner and have been young, in training etc.
Neither your DD nor you should even be asking this.
25 ad 27. Unless BF had offered anything else, it is 50:50 and your DD should be offering and insisting on 50:50.. IF it means parents help one who can't afford, so be it.
If the 27 yo BF really is as lovely as you describe him, he would offer to pay or cover parts of your DD's 50%. But it must be his offer.
So what is 'normal these days'. Your DD should and MUST insist on a place SHE can afford 50% of costs for and make that clear to BF. If he insists on a place DD cannot afford 50% of the costs as it seems here, then 1) he is breaking up with her indirectly and 2) in any even 25 yo women in your DD's shoes would offer to break up if they cannot agree on something she and her parents can afford and is being asked to pay something she cannot afford.
Next: Parents can offer advise to DD to tell BF she will OWE him say 20% she cannot afford for 2 years in 2 years with interest.
Readers, small claim courts are full of this. Your DD should be independent and be able to walk away from this relationship in 1 year etc if it changes. She should not be feeling like she owes her BF. That is for those in marriages who have real, legal reasons for staying even if things aren't great.
So advise your DD to insist on a place she can afford. They also do not have to live together. He can get anther friend to share his expensive flat and she can get a studio she can afford. Simples.