I've had two homebirths, one midwife-assisted and the other unassisted--just my husband, my older child and myself.
It's great that you're going to be able to be there to witness your friend's birth.
My advice is to just be there as help if asked, but if she wants space, then stay out of the way. Let her know that if she wants you to do something for her, to let you know. When she enters transition, and if during that time she starts saying that she can't do it, then speak up and encourage her. Tell her she CAN do it and that it's almost over (b/c when a woman starts saying that and acting that way, it's almost time to push, which is usually enjoyable).
If she needs her back rubbed, offer to do it. If she needs cold cloths for her neck and/or head, get them. If she needs a drink, get one. Remind her to go pee every so often, if you're there for several hours. My last labor was only two hours and fifteen minutes!
I hope everything goes well!
~Tara~