Question : Adding a domain user to a non domain PC

I have a contractor who is trying to add his domain user account to his personal machine. This machine isn't on our domain. When you try and add his domain user account to the local machine it returns an error stating that the trust relationship between the pc and the domain has failed. Is there a way to add his domain user account to a non domain machine?

Answer : Adding a domain user to a non domain PC

Short of joining the domain, not that I am aware of.