Tell them you're sending them positive thoughts or are in their prayers (depending on if they're religious or not), Wish them a easy time with their treatment and a complete recovery. Tell then you're available to listen or drive them if they need it (BUT ONLY OF YOU REALLY ARE). And if you're a really good friend and have that kind of relationship you can sign it 'Breast Wishes'. Believe me, after diagnosis, (and all through treatment) it's great to get well wishes, and thinking of you cards, because sometimes you feel like you've dropped off the face of the earth and friends and co-workers have forgotten you. but you don't want any doom and gloom.
you can also put the following links in their card. they might be of help.