If you think this through, you have to realize that this saying is simply the experience of some people, some of the time. To assert that NO ONE has ever repented and become faithful after being unfaithful doesn't even make sense.

It is also contrary to what the Bible says. If someone has cheated in the past, but later becomes a Christian, the Bible says that person has been changed:

Therefore if anyone is in Christ, he is  a new creature ; the old things passed away; behold, new things have come. (2 Cor 5:17)

Furthermore, those who are already Christians and are unfaithful to their partner, or who fall into any other sin, can be changed by God's Spirit working in their lives:

"But we all, with unveiled face, beholding as in a mirror the glory of the Lord, are being transformed into the same image from glory to glory, just as from the Lord, the Spirit." (2 Cor 3:18) 

The saying "once a cheater, always a cheater" means that change is not possible for these people. This may have been the case before they were Christians, but the Good News is that we can ALL be changed through faith and surrender to Jesus.