Crossing the Tiber is two-way traffic.
I personally know Protestant pastors and elders who were once active and committed Catholics. These men are firmly committed to the truth as well and each felt that the Catholic Church perverted Biblical truth. I'm certain that their testimony would be just as strong and sincere as your former-Lutheran friend.
You wrote:
“I personally know Protestant pastors and elders who were once active and committed Catholics.... I’m certain that their testimony would be just as strong and sincere as your former-Lutheran friend.”
Not at all. There can only be ONE truth. Those who invented Protestantism were not sent by God. They were founders of sects more than 1400 after Christ founded the Church and sent out the Apostles. The founders of sects do not represent or teach the truth. It’s just that simple.