To make threats or coerce someone is.steerpike said:"unlawfully obtaining money"
Welfare is not an unlawful way to obtain money.
Basically: If you don't do what I want, I will injury you financially. That is a threat based on money. The sponsor having to repay welfare is money.
Let's examine an extreme, unusual, but plausible circumstance... Let's say Spouse A makes this threat towards Spouse B, the sponsor. "Leave me alone, give me my divorce, or I will go on welfare and you will have to repay it." B has good connections or people skills and takes written proof of this threat to the government. It goes through various people until it lands on the desk of the Immigration Minister. He orders an investigation into the fraudulent marriage... given the current minister, it seems likely he would. Based on the threats A has made towards B, B is considered more truthful and paints herself as a victim. A gets painted as a criminal. The government declares he was in a MOC and revokes his PR. Yes, he could appeal and would most likely win.... but he does want to have to deal with that? Engaging in criminal activity opens yourself up to great risks.
Is that likely to happen? No. But could it? Yes, especially with the current government. They seem to enjoy martyrs.
He's better off to have a lawyer prepare a cease and desist letter and cut off all contact. File for divorce when legally allowed in his province and move on with his life.