Obviously, being honest is the best thing. But there are a few things to know about underage drinking. It is 100% state law and not federal. Why does that matter? Because depending on the state, will determine the conditions. I am not stating this specifically for your example, but for others who may read this thread and their situation may be different.
For instance, in some states, like wyoming where I live, it is totally legal for a minor to drink alcohol, as long as they have their parents or legal guardians permission, and the parent or legal guardian is physically with them. Whether at home or a party or a neighbors house, etc. Also if the minor is married and their spouse is 21 and is with them. Many states have their own exemptions. It's a state law, not federal. Of course the Feds push laws like purchasing and possession to minors to the states or extort the states by withholding federal funds. By the states are allowed and do make exemptions. Know your states laws.