Would my 20 year old friend get in trouble if we have unopened alcohol in the trunk?

Not if it is your car. If it is hers then they will charge her with possession and she could lose her license for one year.

No, not at all. She obviously should not be talking about intending to drink, or claiming ownership of the alcohol, which is not hers, right? You are twenty-one years old. Presumably, you bought the alcohol and intend to consume it responsibly once you arrive at your destination. Once you have claimed the alcohol, there is really nothing else the police could ask that is any of their business, other than for whatever you were pulled over for in the first place. Some traffic infraction, perhaps? Why, then, would the police be searching the trunk? Politely decline to let them search, if asked.

No. Not with it in the trunk. But, once you get to the party, the adults there can be charged with providing alcohol to minors.

As a preliminary matter, if you are stopped, the police can't just search your car without legal justification or consent. If you provide legal justification or consent for the search of your car, the alcohol is "yours" and you are 21 so you can legally possess and transport alcohol as long as you are in compliance with your states laws.