I think the main thing that is needed is for her to realize her financial situation - realize that money won't suddenly appear on her doorstep just because she is accustomed to her lifestyle. I get the feeling she is refusing to accept the fact that she is now responsible for her actions (with no bailout) and she will have to suffer the consequences of any financial actions (or non-actions in her case). I say this because she doesn't seem to be receptive to the help you are trying to offer.
To me, the best thing to do is to let her assume all her financial responsibilities. I don't know if your wife will agree, but I would tell her not to expect any financial help from you or from your wife. (After all, she has enough money to manage on her own and she is choosing to overspend and there is no reason someone else has to pick up the tab.) (You may end up helping anyway, but your MIL should not expect it and should not plan her budget with that in mind. She is on her own like everyone else.)
You could still break down the numbers for her (her budget/her asset breakdowns and when she would run out of money if she followed her spending patterns.). She needs to face that and realize what she is facing. From what you said, she never was involved in the family finance and she is still playing by the same old rules ("someone else always taking care of it." rule.) I say this because you mentioned that she tries to justify her spending even to you (as if you were her husband who brings back the bacon, who can say "OK, I will give you more money.") If she asks for help in how she could rebudget, then give her advice - options she can pursue etc, but she herself needs to feel nobody is going to bail her out. - It's time for her to rise up to the occasion. Maybe she is still counting on the alimony to arrive on her doorstep, I don't know.
OK, I may sound tough, but my take is she is responsible now for her finance, not you or anybody else and she can do this. I hope she will end up feeling empowered with the notion that she is now in the driving seat for her life.
I did my mom's budget lately and she has enough, as long as she doesn't overspend. Fortunately, my mom realizes that she cannot count on me for any overspending and she lives within her means. I am putting aside some money for her every month, but that is not part of her budget - just gift money. If my mom was like your MIL, I will not help her at all. I live within a tighter budget than your MIL's easily, so I don't see why I should help at all.