Obviously, I don't know the people involved, but I would fear that, if your friend does do the fair thing and leave her DD's 50% to her DGD, he might not accept this and try to put a lot of pressure on her to 'do the right thing' (i.e. the wrong thing, but what HE wants). Even if she does get her rightful inheritance, I'd still hope she wouldn't be targeted to be cheated out of it, once her DGM has died.
However, taking all emotion out of this and purely, bluntly looking at the facts, I would use the following analogy to explain it for anybody having difficulty comprehending:
Bob and Laura are acquaintances and they each do exactly the same job and each earn £4,000 a month for it; and each of them has regular bills totalling £1,000 and is left with £3,000 discretionary spending.
The only difference is that Bob's employer pays him his full monthly wage on the 28th of each month, whereas Laura gets half of hers on the 14th and half on the 28th.
What this man is doing is the equivalent of Bob going to Laura on the 14th and eyeing up her £2,000 that she's just received. He says "Hey, I have the same bills as well, you know! You've just been paid twice as much as you need to pay your bills, whilst I've got nothing at all. It's only fair that you give me your 'spare' £1,000 that you don't need, as your bills are already covered, so you don't need it, and mine are still outstanding whilst I don't have a penny to pay them with."
So Laura feels emotionally bullied/guilted into handing Bob half of her £2,000 and is left with no spare money. Two weeks pass by and Bob gets his £4,000 wages. He doesn't see any reason whatsoever why he should give any of it to Laura - even repaying her the £1,000 that she gave him - as she was lucky and was already able to pay her bills a fortnight previous, without even having to ask or rely on anybody else.
Meanwhile, Bob now has a spare £4,000 which is all his to use exactly as he likes. He enjoys spending it all on lots of lovely treats, which go some way towards making him feel a bit less sad about how very much luckier and far more privileged in life Laura is than he is.