The forgiveness was always there unasked and unearned.<quoted text>
Of course committing adultery one time makes someone an adulterer. An adulterer is someone that committed adultery. There is no number of times required. It only takes once to no longer be a virgin. It only takes one murder to be considered a murderer. Just because they may not do it daily does not mean they weren't a murderer.
But yes, if he repents and "sins no more" then he will be forgiven of that act.
This is not the same as "righteousness".
The litmus test given in Matthew is far different from your soteriology.