OP, did the aunt ever actually state that your friend and his sister would inherit precisely 25% of her estate each?
Everything you've written is ambiguous and, as a result, the whole thread is a game of Mumsnet Whispers; people believing what previous posters have misread or inferred, and discussing that as if it were true.
Friend and his sister, as the closest relatives of the woman were also told that they would inherit.
She told them they would inherit. They did inherit.
They were expecting 50% of the estate (c. £300k) between them.
They expected this. Why? Because she'd explicitly stated that this was in her will? That it was her intention? Or because this was their own idea of what would be fair? Or someone else had suggested this? Or some other reason?
What was the date of the will? Had she written one thing and told them another? Had she changed her will to reflect her feelings about the other three people? Or were they making unreliable inferences?
My friend is furious that he was misled
Was he? Explicitly? Implicitly? Ambiguously? In his own imagination?
He can be furious because he believes he was misled, without actually having been misled.
And frankly, so what if he was misled? Unless his aunt explicitly told him that he would receive a certain amount as payment for his helping her in life and £30,000 is less, at a fair hourly rate, than the value of his assistance to her, then he has nothing to complain about.
Is he actually saying that he would not have helped her in life, without expectation of payment?
If that is the case, he should have drawn up a contract and charged her per hour.
Most people help older friends and relatives because they're decent people with a sense of duty. Often there is no estate left to inherit.