I think the both of you are conflating the sanctification of the Spirit and the process of sanctification (often called progressive sanctification). The two are not the same.
In the Monergist Ordo Salutis sanctification comes after justification. Why? Philippians 1:6 sheds some light on this:
Philippians 1:6 6 For I am confident of this very thing, that He who began a good work in you will perfect it until the day of Christ Jesus.
The Apostle Paul expands on this a bit further in Philippians 3:
Philippians 3:12-14 12 Not that I have already obtained it or have already become perfect, but I press on so that I may lay hold of that for which also I was laid hold of by Christ Jesus. 13 Brethren, I do not regard myself as having laid hold of it yet; but one thing I do: forgetting what lies behind and reaching forward to what lies ahead, 14 I press on toward the goal for the prize of the upward call of God in Christ Jesus.
Once we become a child of God, through the sanctification of the Spirit and the new birth, we are just like John Bunyan's character "Pilgrim" in Pilgrim's Progress. We are now disciples of Jesus Christ and are in the process of being conformed to His image. In other words, we are in the process of being sanctified or made holy. This is the point the Apostle Peter was making:
1 Peter 1:13-16 13 Therefore, prepare your minds for action, keep sober in spirit, fix your hope completely on the grace to be brought to you at the revelation of Jesus Christ. 14 As obedient children, do not be conformed to the former lusts which were yours in your ignorance, 15 but like the Holy One who called you, be holy yourselves also in all your behavior; 16 because it is written, “YOU SHALL BE HOLY, FOR I AM HOLY.”
We currently are imperfect disciples. We still sin and fall short of what God expects of us. Someone once wrote, "We are not sinless, but we should sin less." The Christian life should see us becoming more and more like Christ as we mature. Once again, the Apostle Paul offers insight:
Ephesians 4:22-24 22 that, in reference to your former manner of life, you lay aside the old self, which is being corrupted in accordance with the lusts of deceit, 23 and that you be renewed in the spirit of your mind, 24 and put on the new self, which in the likeness of God has been created in righteousness and holiness of the truth.
In summary, the Reformed or Monergist Ordo Salutis (order of salvation) recognizes the process by which the believer becomes more like Christ. That is why it is placed directly before the final part of the Ordo Salutis, glorification.
In the Monergist Ordo Salutis sanctification comes after justification. Why? Philippians 1:6 sheds some light on this:
Philippians 1:6 6 For I am confident of this very thing, that He who began a good work in you will perfect it until the day of Christ Jesus.
The Apostle Paul expands on this a bit further in Philippians 3:
Philippians 3:12-14 12 Not that I have already obtained it or have already become perfect, but I press on so that I may lay hold of that for which also I was laid hold of by Christ Jesus. 13 Brethren, I do not regard myself as having laid hold of it yet; but one thing I do: forgetting what lies behind and reaching forward to what lies ahead, 14 I press on toward the goal for the prize of the upward call of God in Christ Jesus.
Once we become a child of God, through the sanctification of the Spirit and the new birth, we are just like John Bunyan's character "Pilgrim" in Pilgrim's Progress. We are now disciples of Jesus Christ and are in the process of being conformed to His image. In other words, we are in the process of being sanctified or made holy. This is the point the Apostle Peter was making:
1 Peter 1:13-16 13 Therefore, prepare your minds for action, keep sober in spirit, fix your hope completely on the grace to be brought to you at the revelation of Jesus Christ. 14 As obedient children, do not be conformed to the former lusts which were yours in your ignorance, 15 but like the Holy One who called you, be holy yourselves also in all your behavior; 16 because it is written, “YOU SHALL BE HOLY, FOR I AM HOLY.”
We currently are imperfect disciples. We still sin and fall short of what God expects of us. Someone once wrote, "We are not sinless, but we should sin less." The Christian life should see us becoming more and more like Christ as we mature. Once again, the Apostle Paul offers insight:
Ephesians 4:22-24 22 that, in reference to your former manner of life, you lay aside the old self, which is being corrupted in accordance with the lusts of deceit, 23 and that you be renewed in the spirit of your mind, 24 and put on the new self, which in the likeness of God has been created in righteousness and holiness of the truth.
In summary, the Reformed or Monergist Ordo Salutis (order of salvation) recognizes the process by which the believer becomes more like Christ. That is why it is placed directly before the final part of the Ordo Salutis, glorification.