Just let her talk, cry, rant and rage.
Be normal, be yourself, have a laugh with her.
Don't have a sad face and be all 'oh dear, you poor thing'.
Be smiley and confident and don't walk on egg shells.
Tell her she looks gorgeous and how amazing she is as her confidence will have taken a massive bashing.
Bring her some flowers, make her laugh through her tears and tell her to be strong.
Don't worry about 'how to be', she just wants a listening ear and support.
When it happened to me it was my friends who saved me. They laughed with me and cried with me, made me cups of tea and didn't turn a hair when I let rip.
I didn't want people around me who were self concious or who felt awkward.
When something like that happens to you, you very soon realise who your real friends are and they're like pure gold and very precious.
She'll be feeling as if she's been hit by a train. She'll be in shock and will be running on adrenalin.
Don't be timmid, she'll need you to be strong. Maybe offer to go supermarket shopping with her. It sounds silly but I couldn't do a propper shop for about 3 weeks after my H left as I felt so vulnerable and my eyes were so swollen I could hardly see and in Sainsburys all I saw were all the things I used to buy for him and I'd just cry and go home.
Thank God I'm not like that anymore, I tell you, unless you've been through it yourself (like most things) you cannot begin to imagine what it'like.
Be there for her, text her often, just to say "are you ok, wanna meet up for a tea ?" Most people won't do that as they won't have a clue what she's going through and some of them won't really care. But you do care and well done for wanting to help and coming on here to ask how.
You're a good friend x