Yes it is legal, but if he was convicted of 2 crimes before--especially crimes of dishonesty like forgery or theft or burglary, it makes him a less credible witness. Also, because he is probably doing it to avoid conviction of a 3rd felony, that also makes him less credible, So his job is to testify for the police. Then the criminal defense attorney says, "wouldn't you do anything to stay out of jail for a third felony, such as lie and make up a story about my client?." A pastor or eagle scout would make a much more credible witness.
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