I hate it when people tell me they are sorry. It just doesn't make any sense because they don't really understand. So, just tell her that you don't (or maybe you do) understand and that even though you'll be there for her and if she wants to talk you're there. Just the fact that you're there for her is enough. Well, I don't really know but it was for me when my best friend died. I don't know about trying to cheer her up but try doing things that she likes. For example if she likes shopping, go shopping. Hope this helped!
Just being there for her says a lot. A shoulder to cry on, a sounding board, something or someone to hold on until the "weather" clears. Words are empty and carry little weight during such times. Actions, on the other hand, are priceless and retain their true value. Reverence, remembered.
I've lost both my parents. Before that I to was at a loss for words. I found truly that just a simple "I'm sorry" goes a really long way. You might also offer to help write thank you notes after the funeral. If you need some ideas, check back here I'll give you some. Also, a big hug helps as well. Oh most important, pray for her. She needs it now more than ever.
Well first I'm very sorry for your best friend's loss. But to answer your question, there really isn't anything specific to say to her at this time. Just don't ever say that it's going to be okay, because no matter what, it won't. The most important thing to do is to be there for her, to cry with her when she cries, to listen to her. Make sure she gets back into her normal routine even though it will be so hard to. Psalm 34:18 says "Jehovah is near to those that are broken at heart; and those who are crushed in spirit he saves.” Always remember to pray to our creator, because he cares for all of us and he wants to help you and your best friend.