This is insane.
At 18 if he's NEET then he can claim UC as a job seeker.
He should be helping out with the utility bills/food where he lives with his mum. Maybe the rent too, if she rents, but he can't claim housing costs unless he's on the tenancy and his mum may not want to do that because she may want him to move out.
He can afford Lego and games so he's not as broke as he's making out.
If he was chipping in something towards the household bills his mum wouldn't be so skint either. His mum will be getting extra UC for her health issues if she can't work.
Get onto "entitled to" website and input some hypothetical details for this guy and his mum, separately because he's 18 so they're now two separate households regardless that they live together. So DD can see the kind of income these two may have.
They probably DGAF about the sheets instead of curtains at the windows, they're obviously poor and living like poor people do.
Your DD isn't poor because you all work your socks off, she's so far been sheltered from that side of life and she's shocked by what she's seeing. She needs help to realise that she's not responsible for other people and their life choices.
I'm not suggesting that they're well off at all, but if she's under the impression their sole income is £50/wk or something and can't afford food, she needs educating that's not true.
At the moment she's in danger of viewing him as someone she needs to "save" and having a misplaced sense of responsibility for his welfare. Rescuer syndrome isn't something she wants to develop. It'll make it harder for her to break up with him when she gets bored of his lazy ways, because she'll feel guilty she's not helping him out any more. Developing a rescuer mindset will see her at risk of taking up with one no-hoper after another, it'll ruin her life because her standards for potential partners are too low.
She's also in danger of being used as a cash cow by him. He'll be spending all his money on things for himself and she'll be paying for all the dates and holidays etc. They've known each other 5mins and he's already playing the "I'm so skint, woe is me" card, whilst doing fuck all to change his situation.
Long term DD should find someone better matched to her personality ie someone with a work ethic. This guy is a cocklodger in the making.