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It depends how badly he needs his licence. If he is convicted he will likely receive a driving ban and a fine.
If he intends to plead guilty then a solicitor/barrister can attend and put forward mitigation to keep the ban/fine as low as possible but he can attend and put forward his own mitigation.
If he intends to plead not-guilty then it is best he does instruct a solicitor.
It is quite hard to avoid a conviction for drug driving, he'd need a defence. E.g. he wasn't driving or there is some reason why the drug testing wasn't correct. There are procedural issues you can take such as if the correct procedures for taking blood were followed however in my experience they rarely work.
If he wants to instruct a solicitor to go through all the papers and speak with him then they can advise whether there is any potential defence. There are "specialist" road traffic solicitors you can find on Google who promise the earth but in my experience they aren't actually very good and often overcharge.
You get better value for money by simply instructing a local criminal solicitor. You can find one here: https://solicitors.lawsociety.org.uk.
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