It may not be legal, but police or confidential informants use alcohol and drugs all the time when they're doing a buy and bust. Police can use trickery and can do illegal things to ferret out crime. For instance, it isn't illegal for police to possess controlled substances when they are using them in a crime investigation. Otherwise, in the common situation of police involved in a drug deal, someone could see whether someone was a cop by passing them a joint and seeing whether they smoked it. Nor does a policeman have to be honest in that situation and admit he/she's a cop if asked.
If they got a minor drunk so he was too delirious to know what he was doing if he sold the cop/CI drugs, perhaps you'd have a defense, but I don't think this is what you're asking. You seem to be asking whether a cop/CI can have a drink or two when doing a fake "buy and bust" drug deal with a minor to trick the minor that he's not selling to a cop by making him feel "comfortable", and the answer, I think, is "yes he can".
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There are a number of factors that will determine the answer to your question. If the officer was giving alcohol to the minor in an attempt to alter his state of mind ot commit a crime then yes it is illegal. However, if the minor has a propensity to use the alcohol and sell the drugs anyway then the investigtor's actions may be valid and hold up in court.
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