Although Millie and Squirrel got along well, Harrison knew that her house wasn't the proper place for a squirrel to live.
"He is not meant to be kept as a pet," Harrison wrote in the video's caption. The Dodo hasn't been able to verify whether or not the squirrel was successfully rehabilitated, but Harrison added in the caption that she planned to release him "as soon as possible." Hopefully he's out in the wild where he belongs, burying some nuts and maybe even remembering those happy days with the Harrisons, the folks who saved him when he otherwise would have died.
Have you found orphaned or injured wildlife? It's a tricky situation to handle if you're not an expert, so you should contact a professional wildlife rehabilitation center. You can find one in your local area by clicking here.