You can skip the party and keep the friend. Real ones know the difference.

Attendance is not the measure of a friendship, though it can feel like it when invitations are the main currency. A good friend notices that you turned them down and does not file it as a verdict, because they have more information about you than one Saturday. Someone who treats a single no as a withdrawal is telling you how conditional it was. The difference shows up over months.

Say no to the thing you do not want to go to, then make a separate plan with the person. That pairing is the whole trick: decline the event, keep the person, and do it in the same message.