OK for one Christian to file a lawsuit against an unrepentant Christian?

As recorded at 1 Corinthians 6:1-8, the apostle Paul discussed lawsuits between fellow believers. He expressed dismay that some Christians in Corinth would “dare to go to court before unrighteous men.” (Verse 1) Paul gave strong reasons why Christians should not sue one another in secular courts but rather settle disputes within the congregation setting. Let us consider some of the reasons for this inspired counsel and then touch on a few situations not necessarily covered by this directive.

If we have a business dispute with a fellow believer, we would first of all seek to handle matters God’s way, not our own. (Proverbs 14:12) As Jesus showed, it is best to settle a disagreement quickly before it escalates into a major issue. (Matthew 5:23-26) Sadly, though, some Christians become overly contentious, even taking disputes into secular courts. Paul said: “It is already a defeat for you when you have lawsuits with one another.” Why? A key reason is that such proceedings may well reflect poorly on the good name of the congregation and the God we worship. We therefore take to heart Paul’s question: “Why not rather let yourselves be wronged?”​—Verse 7.

In some cases, a lawsuit may be a simple formality or a legal necessity in pursuing unselfish and peaceful ends. For instance, a lawsuit may be the only mechanism available in getting a divorce decree, acquiring custody of a child, determining alimony payments, obtaining insurance compensation, being listed among creditors in a bankruptcy proceeding, and probating wills. There are also cases in which a brother might feel compelled to countersue in order to protect himself in a lawsuit.*

If such lawsuits are pursued without a spirit of contention, they may not violate the spirit of Paul’s inspired counsel.
In rare instances, one Christian might commit a serious crime against another​—such as rape, assault, murder, or major theft. In such cases, it would not be unchristian to report the matter to the authorities, even though doing so might result in a court case or a criminal trial.

It means to attempt to negotiate a solution, try to work it out, try arbitration first.

Church discipline is found in 1 Cor. 5, and it worked in 2 Cor. 2:6-8.  The remedy is for the church leadership to confront the accused with proper documented evidence, then either disfellowship him, making him a non-member, and subject to a law suit, or satisfy your accusation by paying up.

My family has found themselves in  almost this exact situation.  We ultimately decided not to sue. If he had money in the first place , odds are he wouldn't have lied to us and stolen ours. The fact that my mother forgives  so easily was another factor.  Granted this one took a bit longer, but I don't blame her.  She got there and eventually I did too. 

Paul's letters are not Gospel. Paul was never a disciple or an apostle of Jesus, and all 3 different versions of his Damascus RD. story are fake. He wrongly thought an Apostle was greater than a Prophet which is why he chose to call himself an apostle.

The fool has broken the law of the land, has he not? 

Romans 13:1-2

NONONO...BEST TO SPEAK WITH THE PASTOR FOR SOME HELP.

It means you don't sue them, you just ignore them or be mean to them. I can't figure out the similarities of the pagan and tax collector.

Screwing other people over is what christians do best.  
They've already been proving that for two millennia.

Ok by that logic I can sue anti gay Christians. They lied when they said they wouldn’t judge but they do. I can do the same to Christians who gossip or cheat etc.