It may be said to him:

We do not concede the unrestricted claim that whatever depends on something else cannot be necessary in itself. Rather, what depends on an efficient cause cannot be necessary in itself. The whole’s dependence on each of its parts is not dependence on an efficient cause, but dependence on something else—like the dependence of existence on quiddity, if the two are assumed to be distinct.

It may also be said:

When you say that the whole is contingent, do you mean by “contingent” that which depends on an efficient cause, or that which depends on something else?

If you mean the former, your claim is false. If you mean the latter, why cannot that which is necessary in itself and does not depend on an agent be contingent in the sense that it depends on something else, though not on an agent?

Thus, the very argument he presented answers his own claim here with even greater force. For the dependence of the whole, which is necessary together with its parts, on each of those parts does not negate its being necessary in itself—that self being the whole together with its parts. By contrast, the dependence of existence on a quiddity distinct from it entails that necessary existence depends on something that is not included within it.