Well I'm going to come at it from a similar situation to the friend. I was given a cat in Year 7. Loved her and cared for her, but she will be the only cat I will ever have. (I'm not a cat person) When I moved across the country at 24, my parents and I decided that she would stay with them because she was old (at that point 13 years old) and I would be renting. Five years after that my parents were moving and the area they were moving into doesn't allow cats (issues with cats in general killing local wildlife, so no cats are alive) At that point she was 18 years old and there was no possibilty of moving her across the country) We tried everything to rehome her, and it was looking like we were going to have no option than taking her to the pound. Thankfully a friend stepped up and took her for us. We were lucky enough that I was able to be there when she took her, and it was horrible. My poor cat cried and cried, and I bawled my eyes out. I asked how she was at first but it honestly hurt too much to keep asking, and I didn't want to bother my friend too much. She died about six months ago and my friend messaged me on Facebook to tell me.
Yes you should tell her. The number of people that are saying no horrifies me. You don't have to get involved back in her life if you don't want to, but it will not hurt you to take two minutes out of your day to message her and say 'I'm really sorry, but Cat has passed on' When you rehomed cat for her you were happy to let her back into your life and she flaked out. What's changed now?