Talk. Try to understand what it's like for her. Listen. She might not say much but she is speaking volumes. Be there for her to catch the tears as they fall, her heart as it breaks. Because the day to day agony of HG will end, and the days that follow will be flavoured with the compassion and kindness that you showed her when she was ready to fall.
HG is a very very bad version of morning sickness. Really, superbad. Mothers can become ill very quickly if they don't consume any liquids for a few days, and they really do need to seek help. And maybe now that Her Royal Highness has kindly shed some green-tinged light on a serious condition that affects 1% of expectant mothers.