Cookie doesn’t love me anymore

He may be feeling some sadness at leaving his friends, or he may be tired from the stimulation. Give it a few days. Get back on a routine, and make sure he's getting enough exercise and socialization with other dogs.

I had a similar experience once. We went away for a three week trip, and Mia clearly was under the impression that she was being rehomed. When I came back, she didn't even recognize me. She looked at me from behind the legs of the woman she stayed with, sniffing the air cautiously, when suddenly her eyes got big and she jumped on top of me, not letting another dog near me. It does hurt, even to think about now. It happened during a particularly difficult time in my life, so I set about transforming our relationship, which really meant reclaiming the relationship we had prior to the changes. It took time, but it was worth it.

He may just be exhausted...like a kid home from sleep-over camp.

I think he just needs rest. He’s exhausted from the change of routine and being more active. Just let him be, he’ll come back to you.

Dogs, like people, are pack animals, and he had a marvelous time! Perhaps in his mind, he felt he found a great new pack that accepted him, and there was always something to do and someone to play with. Sometimes only-lonely children go through this too; they visit their cousin's big family and it's so new and different, and darn it, Mom, why do I have go home? Can I stay all summer? 🤓

Maybe this coming weekend, see if your friend will allow you to visit for a 1 or 2 hour play date (or drop him off if she's not having visitors over due to covid). This should brighten his spirits, and the visit isn't so long that he gets the idea it's permanent. Take him straight to the park or for a walk afterwards. See how he acts after a long nap. If he slumps back into a long funk, you might consider getting a puppy.

We had several occasions thru the years when we boarded our girls for a week or so. It always took a few days to return to happy normal.
The first time I was pretty concerned. Did they prefer living there to us? But, in a short time, things were as they were.
Whether they just thought we had left them behind or just liked the novelty of the new environment and company, I felt better the next times, knowing we'd get back to where we were before.

Bobby is always tired the next day when he gets back from playing all day at the doggie daycare.
He loves it there and doesn’t even look back when we drop him off. All those doggies are pretty fun and exciting, way more exciting than me! 😂
He plays there 3 to 4 times a month and all we have to do say one word and he’s ready to go. I just can’t take it personally. 😉When he is boarded for several days it takes him at least a full day or 2 to recover and be back to himself.