Shizel likes her magic, so she may have had run-ins with Ventuswill in the past! Studying or trying to understand what she would have considered magic in her world, etc... But Shizel is also a bit of a dick so she would have kept it formal and precise -- though perhaps there was some moments of tenderness few and far between... (because gasp shizel has a daughter somewhere)

Either way, it's easy to believe that she would have been impressed with Venti's magic enough to think her at least a slight asset on the job.