Visitor is allowed if he/she is invited.

Once I and one colleague was flying Business. He got upgraded to First. After meal I was invited by him to have a drink in First.

CX will also allow visitor from EY/BUsiness to sit in First with their loved one (not ones, one only) to have the meal together if request in advance and ISM allows. But the visitor will only allowed to have the food in their original class. However if there is VIP (the real VIP not CIP or Diamond members) sitting in First, this would not be allowed unless VIP's own guest visiting. Instead the VIP would have the opportunity to have any visitors he wants as sometime they need to talk about serious business.

In one word there is no such thing that any visitor is not allowed in First. One visitor is allowed if invited and have ISM's permission.