It sounds like the problem may be "non-conviction data"...not all arrests/charges are pursued by the prosecutor, but the records remain--other agencies besides the courts keep records that may be reported on background checks. If he hasn't already, he should start with a WATCH report:
Also, the Washington ACLU has some great guides about how to obtain and understand criminal history reports, and understanding employment rights relating to criminal history report:
If those aren't enough, he should consult with an attorney about getting this all cleared up.
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