I am going to move this question to "federal crime" because it really depends on the application of federal firearms law.
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The answer depends on state law. If the state that charged the person with the drug crime classifies a deferred adjudication as a felony conviction, then he would be prohibited from possessing a firearm under federal law. He could also be prohibited from possessing a firearm under state law. Many states ban people from possessing firearms after felony convictions.
If a deferred adjudication does not count as a felony conviction under the law of the state where he was charged, then he was not a prohibited person under federal law, and probably not prohibited under state law either.
He clearly needs an attorney either way.
My answer to your question is based on the facts that you provide in your question. Additional factual details about your situation could change my answer completely. The law in inherently uncertain and always subject to change.
He would need to check the state gun possession laws to see if possession of a gun by a felon is a state crime. It is a crime under the federal system so theretically the feds may get involved and he could have additional federal charges but the feds typically do not adopt it unless they are notified by the state prosecutor or a federal agency was involved in the investigation. Either way, he is looking at serious some serious charges and should seek an attorney.
18 USC Section 922(g)(1) prohibits the possession of a firearm or ammunition by a person who has been convicted of a crime that is punishable by more than one year of imprisonment. 18 USC Section 921(a)(20) states that the term "conviction" is defined by the law of the jurisdiction in which the proceedings were held. Therefore, if the person is considered to have a conviction for an offense that is punishable by more than one year of imprisonment (subject to the limited exceptions in 921(a)(20)(a)) pursuant to state law, then the person is prohibited from possessing a firearm or ammunition pursuant to federal law.
Joshua Sabert Lowther, Esq.
NATIONAL FEDERAL DEFENSE GROUP