Captain M wrote:
Janeway broke the prime directive too. She reasoned that Species 8472 would be a real threat to the entire Galaxy once the Borg were out of the way and therefore she gave the Borg an advantage in exchange for safe passage.
If the prime directive was upheld, Janeway would have turned back and left them to it.
Not sure that it would've been seen as a Prime Directive violation as the Federation, as well as all the worlds between it and the Delta Quadrant, had a stake in the outcome.