Sorry it's so long.
FI has a 12 year old Great Dane who has been by his side since he got out of high school. She's been with him through death of his best friend, heartbreak from his exFI, and through an "I hate the world" phase.
Since I've been around (almost 3 years), she has been chewing on the pad on her back foot. She got some kind of cut a while back, and he took her to vet after vet and nothing worked. She kept chewing through bandages. We tried the lamp shade, but she tore that up too. It's gotten to the point that it's habitual and she does it for no real reason.
She has major hip problems (on top of baby-ing the foot that she chews on), so she's very clumsy and shakey on her feet. For the past couple months, she has had a hard time making it out of the house before she looses control and poops all over the house. It's happening like 4+ times a week now.
A couple of weeks ago it was a beautiful 75 degrees outside, so we left her out while we left for a few hours and she chewed up the doggie door trying to get in. She thinks that she has to be inside all the time now even though she used to be an outside dog.
I asked FI the other night after she had an accident "what are we going to do about her?" and he ignored the question and changed the subject.
I don't know at what point this turns to animal cruelty. He doesn't want to see her completely helpless, but he doesn't want to let her go.
Advice please!

Re: Dog Lovers, I need advice please. Kind of sad.
He doesn't get to do what's easiest for him.
He needs to do what is best for her.
That's really tough because nobody is ever "ready" to say goodbye to a beloved pet, even when they know it is the best thing to do. I am sorry you are struggling right now and the doggie is not at her best either.
I really don't have any advice, that is a hard situation to be in considering how much he and his pet have been through together. All you can do is be supportive and sensitive to his needs, it will be really hard to let go even if things get worse.
Quality of life is the issue here. Is she happy? Does she seem miserable? Is her health affecting her quality of life, or is it just inconveniencing yours?
If she seems unhappy, uncomfortable, and overall miserable, then I would make preparations to put her to sleep.
That is really sad. I'm sorry to hear about her. I agree that it comes down to a question of quality of life for her, as opposed to what your FI wants or doesn't want.
If you haven't already, I would suggest taking her to the vet for a full evaluation and see what they say. If the vet recommends putting her down because of her mobility issues, or other issues, then maybe it will mean more coming from a doc for your FI. A little harder to avoid the truth, you know? Years ago, I remember my step-uncle's elderly dog chewing on her front pads and the vet said it was basically her losing her mental capabilities. I can't remember the name of the exact issue.
Regardless, I'm sorry you have to face this. It sucks knowing you might lose a beloved pet soon.
All of this. I am sorry you have to go through this.
She doesn't seem miserable. She lays around all day and barks at any "bump". She has a Boston Terrier that will practically lay in her mouth so she can play with him. We can get her riled up by having howling wars. That's always fun. But for the most part she is like a baby: all she does is eat, sleep, and poop.
I think I will suggest that we take her to a vet and get a professional opinion.
Thanks for your suggestions.
Aren't they adorable?! He lays right by her side all day!