These things can get involved, but usually you file for credit in resident state for taxes you paid to your non resident state. In your case the nonresident state is now your resident state. Since your client moved, you will have to file PY state return for each state, and I'm not sure you can file for a credit on a PY state return. I would check if you can file a credit on a PY state return in the two states your client has. Then try for the credit if either state lets you. I would have to do more research if I had a client in your situation.