Hi
As you had legally divorced your ex-husband, your previous marriage to him is currently legally null and void. You now cannot claim to be married to him using the old marriage certificate.
You must either remarry him or live with him for 12 continuous months to be able to sponsor him as common-law
1. The OP can expect a "rough ride" for a future sponsorship if there is a re-marriage. Read the following from Regulations.
New relationship
4.1 For the purposes of these Regulations, a foreign national shall not be considered a spouse, a common-law partner or a conjugal partner of a person if the foreign national has begun a new conjugal relationship with that person after a previous marriage, common-law partnership or conjugal partnership with that person was dissolved primarily so that the foreign national, another foreign national or the sponsor could acquire any status or privilege under the Act.
2. I would suggest she search CanLII
https://www.canlii.org/en/ca/irb/index.html with the document text "new relationship" to see how the IAD treats her type of sponsorship. It will help her prepare.