Most Helpful Guy
I wouldn't go as far as an ultimatum but would say "I am going, are you going to come - If you need a day or two to think about it that is okay". I wouldn't hang around to wait for them to change their mind, if they feel that strongly they can come after me.0
Most Helpful Girl
Sooner or later that relationship gets into trouble. You both have to decide go or not.
If they did not want to go i would simply go by myself.1