Sounds like your client may have been telecommuting from Canada. Have you considered whether Canada will have the first right to tax the compensation?
In any case, whatever method allowed under T777S is irrelevant for US tax purposes. All income, deductions, and credits must still be computed in accordance with US tax law.
Depending on the length of his presence and level of activities conducted by your client and his other colleagues in Canada, your client's presence and activities may also have created PE risks to the employer - many a time, corporate tax and finance teams, which have the expertise to review and mitigate such exposures, are blindsided by flexible work arrangements organized by line managers and HR.
Still an AllStar