Never may either precede or follow a tensed auxiliary.
The ordinary position, as you know, is after the auxiliary.
He will never agree.
I have never liked sushi.
She will never be mine.
But in parallel clauses like this, never is usually placed before the auxiliary; this throws the stress onto the auxiliary and thus emphasizes the contrast in tenses.
He has not agreed, and he never will agree.
She is not mine, and she never will be mine.
I don't like sushi, and I never have liked sushi.
Note that the parallel clause need not occur in the same sentence; the speaker may ‘pick up’ a phrase to parallel in the previous sentence.
A: John has never agreed to proposals of this sort.
B: And he never will agree to them, trust me.
The displacement is not obligatory. When the second clause is full, never may occupy its normal place; in this case, never bears the stress.
okHe has not agreed, and he will never agree.
okShe is not mine, and she will never be mine.
okI don't like sushi, and I have never liked sushi.
But when the auxiliary stands alone, with what follows it (the complement and/or the main verb) ellipted, the never must precede the auxiliary.
okHe has not agreed, and he never will. ... but
∗ He has not agreed, and he will never.
okShe is not mine, and she never will be.† ... but
∗ She is not mine, and she will never be.
okI don't like sushi, and I never have. ... but
∗ I don't like sushi, and I have never.
∗ marks a usage as unacceptable
† Note that in clauses where the lexical verb is be acting as a copula (linking verb), the be cannot be ellipted; these are both ungrammatical:
∗ She is not mine, and she never will.
∗ She is not mine, and she will never.
But when the lexical verb is be without a complement, in the sense “exist, occur”, it can be ellipted, and the rules I describe above apply:
okThis must not be, and it never will be.
okThis must not be, and it will never be.
okThis must not be, and it never will.
∗ This must not be, and it will never.