Q. Do I need to do an amended return for Colorado?
A. Probably not. I believe CT tax was mistakenly withheld, since he did not physically work there. You need to file a non resident CT return for a full refund. You may need to attach a letter of explanation.
However, if he occasionally worked in CT (trips, including training days, to the home office), you are required to pay CT tax on the income for those days. So, in that case, you would need to file an amended CO return to claim a credit for the net CT tax paid.
He should talk to his employer about stopping the CT withholding, to avoid filing for a CT refund every year.
