It's interesting and PIP is non means tested extra benefits to help with disability needs. So mum can justify using some of it for additional activities or support that he needs but not for living costs. It is likely it's a large weekly amount far in excess of the £60/ week she suddenly can 'find' for him.
It's outrageous that he cannot access or use any of that precisely for his disability needs- let alone be given none of his other income to buy food when he's out or staying at a friends house. No way should your DD have to buy his food as his mum is withholding access to his living costs income.
I suspect she is already claiming living costs means teated benefits for him (that UC would replace ) as she gets HB for him , so I suspect he gets ESA etc . That's why she doesn't want him to make a UC claim - as both of them can't claim for him. He essentially already has a claim and is in receipt of it but she is managing it on his behalf.
She should have a separate account she puts his money into and be able to evidence what it is spent on. That's his food in the cupboard too, it's just that mum uses his money as well as hers to buy it.
This is why it's a safeguarding of adult at risk, she's withholding his benefits such that he has no money to buy food, nor pay for activities, when he is not with her.
Please support him and look up local authority safeguarding referral process. It doesn't mean ultimately social workers will seek to remove DWP appointeeship from him - they get the subtleties of an over protective parent who can't loosen the reigns even a little bit- but likely discuss with her and him about mum adjusting how she has previously managed his money now he is more independent and goes out without her as his carer.
If she fails to develop and evolve or comply and continues to withhold or spend his money on herself, then they would report her to DWP and seek alternative person to become his appointee or deputy.
Reporting to DWP would not achieve this, they are a blunt instrument and would likely in first instance ask him if he is happy for mum to be his appointee. Without a SW that's a lot of pressure on him to make a difficult decision.