Thanks Alex,

'За' seems to mean 'to go past, or beyond' here then. I knew about 'она залетела', but hadn't thought of it as 'crossing a border.' In English we can say 'he/she crossed the line' meaning that they transgressed, broke some moral code, fell below expected standards of behaviour. Is that sort of how 'залетела' works in this sense?

I can see how 'пролетел' would need the extra context.