Unlike the relationships you may have with your family or friends, the relationship with your co-worker makes for a more difficult setting to respectively show your support.  In any workplace relationships between co-workers are vary greatly.  Chances are you do not know the person very well, they could be someone who just started working at the company, they could be someone from anther department whom you encounter maybe once a week, or it could be someone who you have worked with for many years now.  In any case, the most important thing to do is to somehow show acknowledgement of the situation; whether it be by listening, showing interest, concern, encouragement, or form of support. 
The smallest expressions can sometimes be the most meaningful.

Not sure what to say?
Not sure what to say?  That’s ok.
"I'm not sure what to say, but I want you to know I care."
"I'm sorry to hear that you are going through this."
"How are you doing?"
"If you would like to talk about it, I am here."
"Please let me know if I can help."
"I'll keep you in my thoughts."
“You are doing a great job fighting this.”
“I understand this is tough.”