I remember hearing in the name of Rav Moshe Feinstein that a criminal who-- by normal and objective standards-- should be invalidated from testifying can still be considered a Kosher witness if his crime is not considered criminal as per subjective societal norms.
Where can I find this Teshuva?
Some extra clues: If I remember correctly, the witness in question had, or was going to testify at a Kiddushin. He had evaded taxes, smuggled or something similar in Europe and was now residing in the United States. Rav Moshe claimed that although a criminal by mid 1900s United States' standards, because what he had done was "normal" in Europe, his testimony was valid.
Obviously my memory can be, and probably is, incorrect in certain respects of the story/Teshuva. I was unable myself to locate any references online.