Use member function of another class without object and avoid static

This question is different from those duplicated ones.

I have 2 classes A and B. B has a member function B::boy that uses its variable b.

A has a member function A::evaluate and in this member function, I need to use B::boy.

The problem now is that :

1) B::boy cannot be static because it needs to use a non-static variable b.

2) I cannot instantiate a object of B in A. Because I have one object of B elsewhere and that stores the variable b that is also useful in A.

Is it not possible to be done in C++?