Hi
It all depends on the age of the children, You mentioned 1 is an adult... how old is the younger one?
Up to the age of 18 your friend can apply for a s 8 contact order ... ( Childrens Act 1989) which will aloow for regular contact to be put on a legal basis.
She will need to get legal advice in order to get this proceedure in motion.
The problem with older children is that they have a serious voice ( under the Gillick competence test) when it comes to court proceedings and if they do not want contact then the Courts usually support their opinion.
As for the elder disabled child I am unsure, as Mrs O suggested, she could do with paying a visit to the CAB, I volunteered with them in my second yr of doing my law degree and they know all there is to know about all matters legal!!
Good luck to your friend
J