For early May you're in the window where you can get a long range forecast from accuweather. I wouldn't trust it day to day but the averages might be insightful.
I can't think of a beach destination she can fly straight to so I would meet in a city for a few days and then move on. IST is appealing but maybe not that warm. Rome seems a good option if you haven't been before, then fly to Sicily or Sardinia or simply drive down to Amalfi/Ravello.