If your insurance company will not allow your client to keep his Montana auto insurance on his vehicles while he is away at school in Calgary then he will likely need to obtain his car insurance there in Alberta, Canada. The insurance companies we work with are not able to offer car insurance for your client while he is a full-time student in Alberta, Canada. The insurance companies we work with only extend coverage to the US.
The Alberta government site states that if a person is not a resident of Alberta but is attending school full time in Alberta they do not have to register their vehicles and change their operator licenses. This is provided that the vehicle is registered in their home jurisdiction in their name. The Alberta government does not however discuss the issue of obtaining auto insurance for out of the area full time students.
It is likely to secure auto insurance that the client will need to register his vehicles in Calgary. For help on obtaining the Canadian car insurance he will need on the vehicle will he is attending school in Canada, I would suggest your client seek help from the student services department of the university he will be attending.
The student affairs office of a college or university normally deals with situations such as helping international students figure out how to legally bring their car over and properly place insurance on it so should be able to offer expert advice on his situation.