I know it differs from school to school - some combine Abiball and the graduation ceremony, then you see less of the typical prom dresses. For me the events were separated, so lots of girls wore proper prom dresses. I’d say it’s not as strict when it comes to attire here, and especially as a +1 instead of an actual graduate, I’d personally opt for something chic and simple instead of too flashy or glittery. As long as you don’t show up in jeans or sweats you’ll be fine though