I went through it with my dad...I didn't talk much to him as I was thinking he needs his rest, etc. I never told him how great he was, how much I loved him and appreciated him. My sister was the opposite...she spent alot of time talking to him and knew so much more than me about him in the end that would never have been talked about if he wasn't dying. I regret how I spent my last few months with my dad...it could have been much more fulfilling. He knew I loved him but I wish I told him how much.
In your case I think you should spend as much time with the person as possible...talk about the old day, the great laughs ye shared. Talk about what they would like their loved ones to do when they pass away and most importantly just be there for the person.
I'm sorry to hear you have to go through something like this...stay strong.