You are wrong. T4 indicates the province where the employer is located. If his employer is located in Alberta, then the T4 should show Alberta on it.
When you file your tax return, you only pay tax to your province of residence no matter if he was in Ontario on December 31. If his province of residence was Alberta at that time, then it should have been filed under Alberta.
Now, do you have driving license from Alberta or Ontario? Address of your bank branch? this is how you would determine your province of residence.