Name change for this. My MIL is an absolute gem of a human. Divorced from FIL when children left school, left with not much, has rebuilt her life and is still working in her later years as a substitute teacher to make ends meet. So caring, fun to be around etc. She lives around an hour away by flight from us, 8 hour drive. She used to visit once a month for a few days, which has become less and less in the last few years (of her choosing and we are ok with whatever she needs). The issue is that her BF of 20+ years is a fully functioning drunk. We are talking wakes up in the morning, has a vodka orange, goes to work in a manual job, comes back drinks more, naps the afternoon away, drinks more, falls asleep in the living room in PJs at 6pm while watching blaring TV every single day. He has no qualms about drunk driving, because his base level is so high he can't feel it anymore. They aren't in the UK, but drink driving is very much illegal here, not like Scotland where it is 0 tolerance, but still most people would be over the limit with more than 1-2 glasses of wine.
As can be expected MIL's BF has poor health. He had to get multiple joint replacements in the last year, is obese, and has such bad blood pressure he almost died last year.
Last week MIL and her BF visited SIL who has step children. He was drunk in front of the children again. They are teenagers, he repeatedly called them ungrateful and spoiled and pointing out how rich they are. He said the 13 year old looked like a prostitute for wearing makeup. They are typical teenagers (can be moody etc), but actually very nice and grounded. SIL has understantably hit her limit of tolerating MIL's BF. She called DH after the visit and said that he isn't welcome in their house anymore and to see what he can do talking to MIL. She is refusing to spend time over the holidays with him. Basically if the children are there, he will not be. MIL enables him by giving him a free roof to live under, he doesn't pay the mortgage, bills etc. We gave them our old car last year which is now his. MIL doesn't buy him alcohol and is very active so encourages him to get up and move, but she doesn't force it.
We don't live close enough to actively monitor the situation. We decided last year that if we visit, we all stay at a hotel, the children don't stay at MIL's house while he is there. It is no fun visiting when there is a comatose drunk man with cooking shows on 10x volume asleep and snoring on the sofa. DH's BIL has alcohol issues and is sober of his own doing, we are very proud of him. It isn't fair that he has to watch and draw boundaries either, and his wife is very pissed off to be part of this (pregnant, tired and they are the closest geographically less than an hour drive).
I don't know what to do honestly. MIL is very proud, doens't like to accept money or help. She doesn't want our opinions here, or to have her children cut her off. Her BF will die at some point from his health, but that could be years. He could kill himself drunk driving, or her, or strangers. We don't live there, we can't report him to the police because we don't know when he is out. DH is going to call his mum's best friend as she just saw MIL for a week and see her thoughts, they are close enough and the best friend is quite open and honest. Other than that I am truly at such a loss over what to do here. It's a sad horrible situation.
AIBU - stay out of it, not your circus not your monkeys
IANBU - We should tell MIL our thoughts and lessen contact if her BF is present when our children are
Open to any ideas generally. Mabe people have experienced similar, though hopefully not.