OK - I think I know what this is about and also think it's a great pity that the solicitor didn't bother to explain further. When you own a house with another person, a very basic trust is set up. The Trustee Act requires 2 signatures when selling such a property. Now normally that wouldn't be problem, you'd each sign and everything would be fine. But in a situation like this, when one owner has lost capacity and the other owner is the first owner's attorney, the attorney can sign for themselves, but they can't sign for the person who they are the attorney for, because that's now only one person signing, no matter how many hats they may be wearing. There is a way round this however. Someone else needs to be appointed a trustee to act for the person who has lost capacity just in the matter of the house sale. And when I say appointed, it's not a "go to court and get appointed", it's more a case of drawing up the appropriate documentation.
And that's what the solicitor should have told your mother.