2 days ago we adopted a shih-zu mix is about 5 years old from a friend. He is adapting very well I think but just won't eat. He will eat a treat but no food.

Help!

Are you giving him the same kind of food the previous owners gave him? Perhaps it's that change he doesn't like. Would he eat before you took him in? If so, then it most likely isn't a medical reason.

Try having the friend you got him from come over and feed him there. Getting a new 'owner' is a big change for a dog. It's like a kid's parents giving him to another family. Maybe he just misses his old house or something.

I say give him a few days more to adjust, but try different things.

I wouldn't be surprised if he is stressed, especially as an older dog. Try giving him food like it's a treat, see if you can coax him into it.
I'd start to worry if he doesn't eat by tomorrow, honestly.