SORRY EVERYONE POSTED ON WRONG BOARDThis is a really terrible situation that I feel embarrassed to even be associated with.
Background Info: FI's mom is BSC. She is the kind of person who refuses to take responsibility for her own life - it's everyone else's fault that she can't get ahead, be happy, etc. She also thinks that, now that her two boys are grown, they literally "owe" her for the 18 years she spent raising them. As you might expect, she is also very emotionally and financially unstable and there is always a crisis that she wants FI to deal with for her [[examples: "Go talk to my landlord for me b/c he's threatening to evict me" or "This guy I'm talking to (who is married) has rejected me and I'm really hurt - go beat him up" or "I need money because XYZ."]]. She will create weird reasons to be mad at FI or I. She has also completely alienated all of her family and FI's deceased father's family, and has fabricated all kinds of reasons why they are not good people and she is the only sane one.
Needless to say, I dislike her very much and gave up on wanting a relationship with her. I'm civil to her and tolerate her when I have to be around her, but that's it - FI and I are living our own life now. FI
does know and admit that his mom is a complete mental case.
Last night, she texts FI at 10pm because her car broke and she doesn't have a way to get to work. We go up there and FI gives her one of our cars to use and he is going to buy the parts and fix her car this evening (FI is a mechanic). I did not go into the other room in her house last night where her dogs were, but when we both got back in the car to go home, he expressed how MAD he was at her for the condition that her dogs were in, specifically the 14 year old golden retriever.
If you're skimming, don't skip this paragraph. This is where it gets sad and disgusting. According to FI, she keeps this poor dog chained up in the house, it's nails are so long that they are curled all the way around under her feet and she can't walk, and she is super skinny... like one of those sad ASPCA commerials. The other to dogs are kept locked in little cages in the room. So on the way home while I am driving, he is texting his mom furiously asking WTF!!! She gave him a bunch of BS excuses like "I can't clip her nails because I don't have a nail clipper and can't afford to take her to a groomer" and "I don't know why she's so skinny, she eats all the time!" and "I keep her chained up so I only have to clean her hair in one room."
I have never had so much rage and hate for a person than I do for FI's mom right now. Obviously I never liked her before, but now I don't think I can ever be in the same room with her ever again. I'm shaking with anger even as I type this.
FI and I were both very upset about this last night, and we didn't discuss in detail what we should do about this because we were so mad. What I would like to do is either A) call the local authorities to report it and they will come pick the dogs up and she gets arrested or

go get the dogs ourselves and take them to the local animal shelter. Taking them ourselves is not a possible option because we live in a 600sf apartment - we are not allowed to have pets, and we aren't home often enough anyway.
I want to try to talk FI into Option B - taking the dogs to a shelter where they can find a better home, but (even though FI has expressed his gross disapproval of her maltreatment of these dogs) I am
anticipating that he will resist this because it would "upset" his mom and cause a major blowup.
Even though he knows his mom is nuts, he still does his best to tip toe around her and take the path of least resistance in order to sort of make everyone's life easier I guess. He stands up for me and himself when necessary so she rarely interferes with our life, but I still think he could be a little more assertive with her than he is. I feel bad watching him try to walk a line of being a compassionate son and at the same time not becoming a victim of his mom's crazy train.
While this conversation has not happened yet, I'm worried it that he will ultimately end up not wanting to make this tough call in order to avoid the blowout. And if he doesn't jump onboard with this plan, I don't know what I am supposed to do. I guess I could "anonymously" call the authorities, but then when they are in the picture taking the dogs and charging her, I believe that FI is smart enough to figure out that it was me who made the anonymous call and I could see him getting mad at me for that.
CLIFF NOTES: FI's mom is mistreating her 3 dogs and I want to get FI on my team about taking them from her possession and putting them in an animal shelter.
HELP ME! Give me some wise words or ideas that I simply cannot see now in my state of anger towards this person!