UK - Fake Diazepam (Valuim) blood test, positive?

[Aprof]

A friend of a friend told me he ordered some 30mg Diazepam pills from a fairly iffy looking website a few months ago to aid in his severe anxiety issues. When the (legit looking) box arrived, he ate a pill immediately. He claimed to have no effect, so ate 3 more. After 2 hours, of zero effects, he ate 5 at once. Again, no effect. So ate 10 at once! Still.. no effect

After absolutely no effects, he came to the conclusion that they were fake.

A day later he was caught by Police on an driving offense and a strip of this so-called 'Diazepam' was found in his wallet - so a blood test was taken. The blood result has come back positive for Diazepam. The police may attempt to add this result to aid in the prosecution of this driving offence.

This guy is going out of his mind with questions. Is this a false positive? Does this guy have insanely high tolerance for Valium? Were the pills just incredibly weak? If the blood test result shows such a low level it be argued that they had no effect on the driving?