In the United Stated, to my understanding, it is illegal for anyone under the age of 21 to purchase, consume, or otherwise be in possession of alcohol. Therefore, would it be illegal if someone who was under 21 (Person A), went to dinner with someone (Person B) over 21 (parents, older relative, date, etc.) and Person B ordered an alcoholic drink and Person A didn't, and when the check came, Person A decided to pick up the tab? Could the police arrest this person and charge him/her with purchase of alcohol by a minor, and subsequently charge Person B with allowing a minor to purchase alcohol (I.e negligence?) And then fine the restaurant for allowing a minor to purchase alcohol?
The police can arrest and charge anyone if they have a reasonable belief that a crime has been committed so: yes.
However, if the question is: can they secure a conviction the answer is a resounding no.
North Carolina law has three prohibitions on minors and alcohol: supplying to, possession of and buying.
Possession is easily dealt with: the minor never possessed the alcohol.
Supplying is also dealt with perfunctorily: the supply went from the restaurant to a person 21 or over.
This only leaves buying. Now, buying something requires a contract (or since the object is illegal, what would be a contract but for that). Did the minor have a contract with anyone? That is, were they under an obligation to pay for the alcohol? No. If the pair had skipped out on the bill, the restaurant would pursue the adult for payment because that is who they have a contract with. Any arrangement between the diners as to how they will split the bill is a private, unenforceable arrangement, not a contract.