The five stages of grief as identified by Elisabeth Kubler-Ross are: denial, anger, bargaining, depression, and acceptance.
Sanders supporters have denied the inevitability of Joe Biden becoming the Democratic nominee. It's as if they are gazing at a gravely and terminally ill family member or close friend expecting them to rally as the nurse is about to unplug the machines and pull the sheet over the patient's face.
Many Sanders supporters have expressed their anger at any target they can find: Joe Biden, the media, the DNC, the voters, even Bernie Sanders himself.
Some Sanders supporters have reached the bargaining stage wherein they issue a demand or propose a "deal", e. g. if Bernie doesn't win the Democratic nomination, I am voting for Donald Trump! or Drop out, Joe Biden, or I'll accuse you of sexual assault!
It won't be immediately obvious when Sanders supporters reach the depression stage, as they are likely to turn inward and become less vocal therefore less noticeable. But if you know somebody there, be gentle and kind. Give them a chance to express their feelings without arguing or making a judgment.
It's too early to think many Sanders have reached the acceptance stage yet. When they do, refrain from taking a "I told you so" attitude.