Heidi, We went thru this with husband's sister. She was only 49 when she passed away after a two year battle with brain tumor. Dale was young and vibrant and had so much life to live. She and my husband were very close; no one wanted to see her go. I think she held on longer and endured more suffering than any one should ever have to because of the family. Even though she was tired of fighting, we encouraged her to be brave and fight to the end. Everyone had the best intentions. But in hind sight, by doing this we put a lot of added pressure on her. I think we imposed a sort of "guilt" on her, making her feel like she was letting us down if she gave up. She didn't want to disappoint us and couldn't find peace as long as she had that guilty feeling.
I know this is hard for your husband and he wants her to fight. But, if she is ready to go, help him realize she needs to be able to do so with a peaceful heart and without the worry of disapointing those that are left behind.
If she is done fighting, I second Amy's advise: Tell her you love her, hold her hand and make sure she isn't in pain.
Hugs, Vickie