If your friend is a convicted felon for drug possession, I would bet he has a criminal history that is greater than simple drug possession. In Arizona, first time possessors are usually given an opportunity to clean up in exchange for a dismissal of the charges. (Commonly referred to as a TASC deal.) So, if your friend has a conviction, it leads to the assumption that he has been in trouble previously and was not eligible for TASC or had a larger drug quantity and plead down to a possession charge.
The new offense is unrelated, assuming it is just the Domestic Violence Charge and that he is being forthright with you. It is a misdemeanor and likely being handled in a Justice or City Court. Much depends on what the prosecutor is willing to do and what his actual criminal history is. He will likely be appointed a public defender. If you are not satisfied with that, consider hiring private counsel who can help resolve the situation. www.woodnicklaw.com
This general information does not create an attorney client relationship between the questioner and the author. The internet is NOT a place to get 'legal advice'...although gathering general information is always helpful.