Tell him you will commit suicide if he keeps paying her bills. See who wins.

Is this affecting you financially or just him? Has she got other kids? Who can help? What is her financial situation actually like? This is a hard one to answer without more info.

(1) This is an example of why the treatment of depression or mental illness is best left to a mental health professional. We have no way of knowing if MIL is actually depressed and likely to kill herself or if these threats are a form of manipulation to get what she wants. If it’s purely manipulative behavior with no actual serious depression, I would call her bluff in a heartbeat. (2) Does MIL make consistently bad spending choices that cause her to be unable to pay her bills such as excessive spending on gambling, “recreational” drugs or alcohol or even tobacco, or being a “shopaholic”? If something like this is the root cause of her financial problems, then those problems have to be addressed or no amount of financial assistance will ever be enough. (3) Everyone except MIL has my sympathy in this FML. And it is possible that MIL actually is in dire straits and desperate, but I have serious concerns about how she is getting the financial help she seems to need and why is she in this apparent situation. By the way, before you condemn me for being heartless - I have suffered from depression on and off much of my adult life. I never threatened to commit suicide to get what I wanted and would never do so. I considered suicide when I was at the bottom of the pit (not as a threat). But this sounds more like sick, manipulative behavior to me than depression.

If she threatens suicide, call the cops to do a wellness check. An aggressive threat like that deserves an aggressive response to show you take her word seriously. He needs to stop enabling this manipulation but stopping payments. Threats like this are manipulative and used to torture and extort people. It's abuse, plain and simple.