New comers to Canada

When a married person comes to Canada alone and the spouse left at back home, do we consider the taxpayer married or separate for tax purposes?

Married.

Here is a good summary of how it works:
https://www.howlandtax.com/articles/nonresident-spouse.htm

jpglass’s article referenced {Tax Implications When Canadian Residents Marry Non-residents} is, however, not exactly your clients situation.

There is no question whatsoever that the clients ARE married for tax purposes (since you have not indicated that they provided you a copy of Court/Official Separation Agreement/Divorce papers).

.
Tax Residency, however, is a whole other problematic kettle of fish…

The proper tax treatment is going to need a lot more investigation and documentation and reference to the ITA of both countries and the Tax treaty, and inquiries into Facts of the individual and family,

There have been cases posted in these forums before where the whole family was supposed to be reporting Tax Resident - and others, none of them…