Here is where I place my two cents. In some couples, in this case my parents, my mother is unable to work. She lives on retirement benefits and some savings. My father works and spends like the world is ending. They talk, but my mother can not make my father stop spending money, and it is not worth it to her to separate from him over it. My mother ended up getting separate bank accounts, so she can pay the utilities and taxes out of her income, and my father's spending can't affect those accounts.
I would have absolutely nothing to do with a party like this thrown by my father, because I could not in good conscience accept frivolous spending when I know they were nearly penniless last year because of another frivolous purchase by my father.
The above is my opinion. I would not say anything to my mother, but I would not participate at all.
I'll add my two cents to ladyknight1's two cents.
While Aeris' profoundly insightful post has moved me to deep sympathy for the MIL and the idea of tipping her off, I would still stay out of it and not participate.
For better or for worse, for richer or poorer, this is MIL's husband and she has to deal with him (or not deal with him). In OP's or OP's DH's shoes, if MIL took her anger out on me, I'd refuse to accept it. I'd simply say, "I'm sorry you're upset. But this was your husband's idea. Deal with him."
In a much smaller and less consequential situation in my own family, my dad would let his blood sugar drop (not managing his diabetes terribly well) and the typical behavior of our family is (family here is: my parents, brother and current GF, my DH, my kids and myself) when we go out to eat is no one will take the lead in picking a place out. We dither until Dad explodes because he's starving. You can set your watch to this most times.
Finally one day when my dad was in a rage I just took my family and left. The next day he came to apologize and I appreciated it but I said, "Look, if we're dithering and you absolutely can't take it anymore, just say so and leave. It's just a meal." He agreed and the next time it occured, he grabbed my mom's hand and said, "Bye guys, I'm going home." My mom and brother were stunned. As my dad pulled away he was smiling at me and I smiled back and waved.
My brother didn't agree with Dad's decision or my idea. He said it wasn't fair to Mom. I told him I thought it was very fair to Mom. SHE picked him and has stayed married to him and this is one of non-perks of being married to him (there were more perks than non-perks). I also mentioned that while it would've been nice to have a meal with them if Dad was in a good mood, we'd still get to have a nice meal with just the 5 of us. And maybe we'll all learn to be better organized in the future. Which we did!
Like, I said this was a very small thing but it change a dynamic in the way we interacted with my FOO.
If I were the OP, knowing what I know from this thread, I'd try to get DH on the same page as myself. And my page is to sit down with FIL and say, "We won't tell your surprise but what we know of MIL and your finances means we don't want any part of this or to particpate in any way. We'll take you both out to dinner (or whatever way OP & DH want to celebrate the IL's anniversary) to celebrate at a time that's convenient for you." Cue possible arguing and threats but if the FIL's own son
won't help or participate that may cause him to pause and think.
Even if it doesn't, at least OP & DH can stay out of it. If he goes through with it, MIL will probably be more furious about the debt than why her son and DIL weren't at the party. If MIL tries to direct any rage about keeping secrets at OP & DH, they can refuse to accept it and redirect her to her own husband.
It would take a pretty sick parent to blame a child - even an adult child - for the behavior of his or her own spouse.
ETA: Gramma Dishes I'd just like to add that I'm differentiating anger and blame. I re-read your posts and noticed where you said you'd be furious at your kids if they knew and didn't say anything.