Not so black & white as a yes or no, but my short answer would be No...I don't think he cheated.
Is it possible, yes. But My ex wife worked in a very respected University Lab and they had tests have false positives, samples get compromised etc... several times a year despite stringent rules and safeguards that were "supposed" to be followed. So while it's possible he is guilty, I'm more inclined to say he's not and give him the benefit of the doubt for the time being.
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Just Do It..