This is a sad day for all of you. Putting an animal down is the hardest thing ever. I put my favorite cat down in 2001 and every once in a while I still have a good cry for her. I'm in tears now thinking about my Gina.
I have never allowed myself to have another pet since then and just couldn't even consider it. Its a pain I could never go through again.
I completely understand what they are going through tonight. My thoughts are with them. How brave of them to have the celebration of her life today to make the day one with some good memories too.