Things to say to a sick colleague?

So, a colleague of mine, my sempai and dare-I-say friend, was just put on sick leave after being diagnosed with breast cancer. I am devastated. My mother was diagnosed with the same some years ago, so I'm familiar with the treatment and struggle…

Back in my home country, I know how I would express sympathy for the situation. I know what kind of "get-well" gifts I would get, etc.

… but here in Japan, I've got no clue. First off, I don't know what to say. 「それはひどい」doesn't quite cut it, ね？My Japanese honestly isn't good enough to come up with something heartfelt and sensitive to say, but I would really like to express my sympathies and how much I hope for her speedy recovery (and for her to kick the big C's ass!)

Anyone know what a) is common practice in this case in Japan? b) could be said in this scenario in something like a letter or card?

Thank you. Any advice appreciated.