If your client lacks mental capacity, the mental capacity required for consent to sex is not at a high level, and it should not be assumed that any person lacks it. If your client truly lacks mental capacity to make decisions about sex because they have been assessed as being unable to understand what is involved, and any risks involved for them, then it is important to consider whether they could be helped by tailored sex and relationship education.
This is how they handle issues about consent for their service users! Basically,just work around it.
Somehow I don't think they are much more careful or worried about the consent of prostitutes.
Not just that, but I do believe it leaves the people using this service quite vulnerable as well depending on what their disability is.
I'd rather more effort was put into socialising, developing hobbies and interests ,encouraging relationships to form naturally at events and organised days etc.
You can't fix loneliness with a wank.