Adult severely disabled coming out of education benefits

friend of mine has a severely disabled son who lives with her. He is now 19 so my friend can no longer get the benefits for him it all goes to him, she is obviously worried how she will cope now financially. Can anyone help here? Basically he will now get his own pip and uc and she will obviously loose her entitlements for him. How does it work for her now for help from him financially or is she not allowed to have any income from him to help towards the loss. If she can how does she work out a fair amount as it’s all very daunting to her as it’s also confusing because he’s not mentally capable of dealing with his own money. Has anyone got any experience of this or point me to some forums that might know? So basically want to know if she is allowed to charge some form or rent help for bills or is it not allowed and if it’s allowed how does she work out what would be classed as a fair amount so its not classed as abuse of his money.