There's no such thing as a sinful nature inside of a born again person. Our sinful nature is EXACTLY our physical body. The outer man., this body of death.
If both of you believe this, then no wonder your off base in your interpretations.
The flesh is not the sinful nature but rather as Paul explicitly carefully states "the law of sin" dwells in the flesh.
but sin that dwelleth in me.
18 For I know that in me (that is, in my flesh,)
The material flesh is not sin, but it is the sphere of where the "law of sin" works and dwells.
The "law of sin" is put in direct contrast with another "law" operating in his mind (inward man)
But I see another law in my members, warring against the law of my mind, and bringing me into captivity to the law of sin which is in my members.
The law of his mind was the law of God that his "inward man" delighted in, that his will was inclined to obey (v. 18). The "law of sin" is no more his flesh, then the law of God is his mind. Rather, both mind and flesh are ruled by different principles. However, his mind, is inclined to the law of God, as his flesh is inclined to the law of sin, his mind does not have POWER to over rule the law of sin in his flesh.