This thread just brought me to tears. I had never before allowed myself the reality to think that my mother is going to stop caring for herself in the most personal of ways. I watch her to make sure she eats enough (and other family members, particularly my dad, do as well) and I'm always worried about her falling, but I try not to focus on the long-term aspects of this horrible, horrible way to die because it hurts so much to think about the fact that my wildly intelligent mother can't remember what day of the week it is anymore.
When I see my mom with my daughter I can't help but think, how much of this is she going to remember in a year or two? When will she start forgetting BabyX's name? Or mine? When she talks about parenting with me, she only ever tells stories of my two older siblings and I already think that she's forgotten much of my and my younger sibling's baby and toddler years, which is more painful than I ever would have thought possible. I'm noticing my mother get quieter and quieter, not only because she can't think of what to say or can't find the words but, frequently, because she doesn't fully understand what's going on around her and doesn't want to ask. She loses track of tasks in the middle of doing them, but doesn't want to give up doing anything because that would cut into her independence. It seems like, even at home, she's gone downhill very fast. I can't even imagine the trauma that will ensue for all of us when we finally do have to move her into some other type of care.
All this is to say, others suffering through this same hell have my complete sympathy, and my sorrow. There are just no words to take away the pain of watching your loved ones leave you this way. It is a painful reminder of who they once were to see their shell every day, but without that vibrancy and knowledge and ability which made them who they used to be.
OP, how does your spouse express pain? Emotional pain, physical pain, doesn't matter. I find that in most people, the two are related. If your husband yells or curses when in extreme physical pain, he's probably the type to want to talk and express himself. Some people are brooders, some are fighters. Me, I get very quiet and withdrawn when I'm in a lot of pain. Not grumpy or despairing, just very quiet and reflective. When I'm emotionally upset, I just want to be by myself for a bit. I talk with my husband occasionally about what I'm feeling and thinking about my mother's Alzheimer's, but it's a bit forced and mostly just done so that I'm not completely shutting him out. These conversations are very short, because I find it hard to put everything into words. I don't even talk with my siblings about it all that much, though I'm very close with them. It tends to be more functional conversations, updates about a recent fall or what we can do to help her on her bad days. That's just how I deal with a lot of pain, by absorbing it quietly and then trying to move on. So, to best help your husband I'd start by figuring out what his, for lack of a better term, pain style is and, frankly, asking him what he needs most from you. It might be long walks and conversation, it might be time alone to work through this on his own, being held silently, or any number of different things. If you're just not sure, letting him know in small ways that you're there for him are never a bad thing for a marriage.
Supporting the spouse of the dementia patient is a bit trickier. I'd say that running errands for her when possible and getting her out, giving her someone to talk to about topics not at all related to dementia, are probably the best things you can do. Also, asking your spouse if he's got any suggestions for how you could help his mother both shows your support and will hopefully take his mind off his own feeling of helplessness. Since he knows her better, he might have some suggestions for little things that would make her feel better or less stressed.
Are there any support groups in your area for friends and family of dementia patients?