Bangladesh may recognize over-the-phone marriages but I sincerely doubt that Canada does. However, the only way to find out is to find out what kind of marriage certificate would be produced in Bangladesh and see whether or not that is acceptable proof of marriage in Canada (and it must be in English or French, or translated).
Canada recognizes civil marriages as well as religious marriages so the issue is whether or not the marriage is legal in Bangladesh, first and foremost. If she needs to convert for the marriage to be legal, then that document is required. It would not be for a marriage in Canada.
She will need to show she has full custody of her child in order to immigrate with the child.