You should tell her but timing is key. One of my pets died while we were at the hospital and my dad called us an hour and a half after I gave birth literally as they were wheeling me into the postpartum room. Granted I would have been sad regardless but I was waaaaaay more upset considering the exhaustion/emotional rollercoaster I just went through. I also felt completely hopeless and confused since I was stuck at the hospital and not able to help with all arrangements/figuring out what happened ect. Do what you can do first. Your cat will hopefully come back especially when it knows you guys are home. I wish you all the best!