Being present with someone is the kindest gift of all. Don't think of what to do or what to say, just be there with her.
If you're a religious man, pray for guidance, clear your mind of preconceptions, plans or fear, and you'll be provided in what to do and say when you're with her.
If you're a secular man, look within for guidance, clear your mind of preconceptions, plans or fear and your best self will know what to do and say when you're with her.
Be you. Anything else will be obvious. Don't make her have to take care of your fear and uncertainty, rather than the other way around.
Treat her as the woman she is -- so much more in her mind and heart and body than just cancer. She probably doesn't get enough of that at this point, people remembering she's more than just illness.
Hold her hand. Cry with her. Tease her. Make her laugh. Bring her things she loves. Watch her favorite stupid comedy with her.
Most important, show her she isn't alone through any of this...