I killed a PC’s animal companion at the end of last session, but later realized it should have survived; what are my options?

Notes: I am the DM, my players are a group of 7, levels of 11 or 12.

So, our last weeks session ended with a black dragon releasing an acid breath attack on the only visible enemies in a courtyard. This was a servant, my wife’s ranger PC, and her falcon animal companion. The servant and the falcon failed their Dex saves and took 58 points of acid damage, outright insta-killing both of them.

Going back to look at it as I prep for the upcoming session, I realize the falcon stats were incorrect. It should have 4 times the ranger’s level in hit points. As she is level 11, this would mean the falcon should have 44 hit points. Not only that, but the falcon should have had the ranger’s proficiency added to the Dex save, which would have made them pass the DC 18 Dex save, reducing the damage to 29 instead of 58.

As this was the last thing to happen at the end of last session, I wasn’t sure if I should retcon the hawk to still being alive and conscious, or leave it a pile of goopy acid and note that I will fix it with the next animal companion? What are my options in a situation like this and what are the pros and cons of making those choices?